Docker Guide SUSE Linux Enterprise Server 12 SP3
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Docker Guide SUSE Linux Enterprise Server 12 SP3 Docker Guide SUSE Linux Enterprise Server 12 SP3 This guide introduces Docker, a lightweight virtualization solution to run virtual units simultaneously on a single control host. Publication Date: October 26, 2017 SUSE LLC 10 Canal Park Drive Suite 200 Cambridge MA 02141 USA https://www.suse.com/documentation Copyright © 2006– 2017 SUSE LLC and contributors. All rights reserved. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or (at your option) version 1.3; with the Invariant Section being this copyright notice and license. A copy of the license version 1.2 is included in the section entitled “GNU Free Documentation License”. For SUSE trademarks, see http://www.suse.com/company/legal/ . All other third-party trademarks are the property of their respective owners. Trademark symbols (®, ™ etc.) denote trademarks of SUSE and its affiliates. Asterisks (*) denote third-party trademarks. All information found in this book has been compiled with utmost attention to detail. However, this does not guarantee complete accuracy. Neither SUSE LLC, its affiliates, the authors nor the translators shall be held liable for possible errors or the consequences thereof. Contents 1 1.1 1.2 Docker Overview 1 Docker Architecture 3 Docker Drivers 3 Container Drivers 3 • Storage Drivers 4 2 Docker Installation 5 2.1 General Preparation 5 2.2 Networking 6 Networking Limitations on Power Architecture 7 3 Installing sle2docker 8 4 Storing Images 9 4.1 What is a Docker Registry? 9 4.2 Installing and Setting Up Docker Registry 10 4.3 Limitations 11 4.4 Portus 11 5 Creating Custom Images 13 5.1 Obtaining Base SLES Images 13 5.2 Customizing SLES Docker Images 14 Creating a Custom SLE 12 Image 15 • Creating a Custom SLE 11 SP3 Image 15 • Adding SLE Extensions and Modules to Images 16 6 iii Dockerizing Applications 17 6.1 Running an Application with Specific Package Versions 18 6.2 Running Applications with Specific Configuration 19 Docker Guide 6.3 Sharing Data between an Application and the Host System 20 6.4 Applications Running in the Background 21 7 7.1 A A.1 Working with Containers 23 Linking Containers 23 Documentation Updates 24 September 2017 (Initial Release of SUSE Linux Enterprise Server 12 SP3) 24 A.2 November 2016 (Initial Release of SUSE Linux Enterprise Server 12 SP2) 24 B B.1 iv GNU Licenses 26 GNU Free Documentation License 26 Docker Guide 1 Docker Overview Docker is a lightweight virtualization solution to run multiple virtual units (containers) simultaneously on a single control host. Containers are isolated with Kernel Control Groups ( Control groups ) and Namespace . Full virtualization solutions such as Xen, KVM, or libvirt are based on the processor simu- lating a complete hardware environment and controlling the virtual machines. However, Docker only provides operating system-level virtualization where the Linux kernel controls isolated containers. Before going into detail about Docker, let's define some of the terms used: Docker engine The docker engine is a server-client type application that performs all tasks related to virtual machines. The Docker engine comprises the following: daemon - is the server side of the docker engine that manages all docker objects (images, containers, network used by containers, etc.) REST API - applications can use this API to communicate directly with the daemon a CLI client - that enables you to communicate with the daemon. If the daemon is running on a different machine than the CLI client, the CLI client can communicate by using network sockets or the REST API provided by the docker engine. Image An image is a read-only template used to create a virtual machine on the host server. A Docker image is made by a series of layers built one over the other. Each layer corresponds to a permanent change, for example an update of an application. The changes are stored in a le called a dockerfile. For more details see the official Docker documentation (http:// docs.docker.com/engine/reference/glossary#image) . Dockerfile A dockerfile stores changes made on top of the base image. The Docker engine reads instructions in the dockerfile and builds a new image according to the instructions. Container A container is a running instance based on a particular Docker Image. Each container can be distinguished by a unique container ID. 1 SLES 12 SP3 Registry A registry is storage for already created images. It typically contains several repositories There are two types of registry: public registry - where everyone (usually registered) can download and use images. A typical public registry is Docker Hub (https://hub.docker.com/) . private registry - these are accessible for particular users or from a particular private network. Repository A repository is storage in a registry that stores a different version of a particular image. You can pull or push images from or to a repository. Control groups Control groups, also called cgroups , is a Linux kernel feature that allows aggregating or partitioning tasks (processes) and all their children into hierarchically organized groups to isolate resources. Namespace Docker uses namespaces for its containers that isolates resources reserved for particular containers. Orchestration In a production environment you typically need a cluster with many containers on each cluster node. The containers must cooperate and you need a framework that enables you to manage the containers automatically. The act of automatic container management is called container orchestration and is typically handled by Kubernetes. Docker is a platform that allows developers and system administrators to manage the complete life cycle of images. Docker makes it easy to build, ship and run images containing applications. Docker provides you with the following advantages: Isolation of applications and operating systems through containers. Near native performance, as Docker manages allocation of resources in real time. Controls network interfaces and resources available inside containers through cgroups. Versioning of images. 2 SLES 12 SP3 Allows building new images based on existing ones. Provides you with container orchestration. On the other hand, Docker has the following limitations: LIMITATIONS OF DOCKER Containers run inside the host system's kernel and cannot use a different kernel. Only allows Linux guest operating systems. Docker is not a full virtualization stack like Xen, KVM, or libvirt . Security depends on the host system. Refer to the official security documentation (http:// docs.docker.com/articles/security/) for more details. 1.1 Docker Architecture Docker uses a client/server architecture. You can use the CLI client to communicate with the daemon. The daemon then performs operations with containers and manages images locally or in registry. The CLI client can run on the same server as the host daemon or on a different machine. The CLI client communicates with the daemon by using network sockets. The architecture is depicted in Figure 1.1, “The docker architecture”. FIGURE 1.1: THE DOCKER ARCHITECTURE 1.2 Docker Drivers 1.2.1 Container Drivers Docker uses libcontainer (https://github.com/docker/libcontainer) dle containers. 3 as the back-end driver to han- Docker Architecture SLES 12 SP3 1.2.2 Storage Drivers Docker supports different storage drivers: vfs : this driver is automatically used when the Docker host le system does not support copy-on-write. This is a simple driver which does not offer some advantages of Docker (like sharing layers, more on that in the next sections). It is highly reliable but also slow. devicemapper : this driver relies on the device-mapper thin provisioning module. It sup- ports copy-on-write, hence it offers all the advantages of Docker. btrfs : this driver relies on Btrfs to provide all the features required by Docker. To use this driver the /var/lib/docker directory must be on a Btrfs le system. AUFS : this driver relies on the AUFS union le system. Neither the upstream kernel nor the SUSE one supports this le system. Hence the AUFS driver is not built into the SUSE Docker package. SLE 12 uses the Btrfs le system by default, which leads Docker to use the btrfs driver. It is possible to specify which driver to use by changing the value of the DOCKER_OPTS variable defined inside of the /etc/sysconfig/docker le. This can be done either manually or using YaST by browsing to System /etc/sysconfig Editor System Management DOCKER_OPTS menu and entering the -s storage_driver string. For example, to force the usage of the devicemapper driver enter the following text: DOCKER_OPTS="-s devicemapper" Important: Mounting /var/lib/docker It is recommended to have /var/lib/docker mounted on a separate partition or volume to not affect the Docker host operating system in case of a le system corruption. In case you choose the Btrfs le system for /var/lib/docker , it is strongly recommend- ed to create a subvolume for it. This ensures that the directory is excluded from le system snapshots. If not excluding /var/lib/docker from snapshots, the le system will likely run out of disk space soon after you start deploying containers. What's more, a rollback to a previous snapshot will also reset the Docker database and images. Refer to Creating and Mounting New Subvolumes at https://www.suse.com/documentation/sles-12/ book_sle_admin/data/sec_snapper_setup.html 4 for details. Storage Drivers SLES 12 SP3 2 Docker Installation 2.1 General Preparation Prepare the host as described below. Before installing any Docker-related packages, you need to enable the container module: Note: Built-in Docker Orchestration Support Starting with Docker 1.12 the container orchestration is now an integral part of the Docker engine. Even though this feature is available in SLESSP1 and in SLESSP2, it is not supported and is only a technical preview. Use Kubernetes for Docker container orchestration, for details refer to the Kubernetes documentation (http://kubernetes.io/docs/getting-started-guides/kubeadm/) . PROCEDURE 2.1: ENABLING THE CONTAINER MODULE USING YAST 1. Start YaST, and select Software Software Repositories. 2. Click Add to open the add-on dialog. 3. Select Extensions and Modules from Registration Server and click Next. 4. From the list of available extensions and modules, select Container Module 12 x86_64 and click Next. The containers module and its repositories will be added to your system. 5. If you use Subscription Management Tool, update the list of repositories on the SMT server. PROCEDURE 2.2: ENABLING THE CONTAINER MODULE USING SUSECONNECT The Container Module can be added also with the following command: $ sudo SUSEConnect -p sle-module-containers/12/x86_64 -r '' Note: Note about the SUSEConnect syntax The -r '' ag is required to avoid a known limitation of SUSEConnect. 5 General Preparation SLES 12 SP3 PROCEDURE 2.3: INSTALLING AND SETTING UP DOCKER 1. Install the docker package: sudo zypper install docker 2. To automatically start the Docker service at boot time: sudo systemctl enable docker.service This will automatically enable docker.socket in consequence. 3. In case you will use Portus and an SSL secured registry, open the /etc/sysconfig/docker le. Search for the parameter DOCKER_OPTS and add --insecure-registry ADDRESS_OF_YOUR_REGISTRY . 4. In the production environment when using the SSL secured registry with Portus, add CA certificates to the directory /etc/docker/certs.d/and copy the CA certificates to your system: sudo cp CA /etc/pki/trust/anchors/ && update-ca-certificates 5. Start the Docker service: sudo systemctl start docker.service This will automatically start docker.socket in consequence. The Docker daemon listens on a local socket which is accessible only by the root user and by the members of the docker group. The docker group is automatically created at package installation time. To allow a certain user to connect to the local Docker daemon, use the following command: sudo /usr/sbin/usermod -aG docker USERNAME The user can communicate with the local Docker daemon upon his next login. 2.2 Networking If you want your containers to be able to access the external network, you must enable the ipv4 ip_forward rule. This can be done using YaST by browsing to System Network Settings Routing 6 Networking menu and ensuring Enable IPv4 Forwarding is checked. SLES 12 SP3 This option cannot be changed when networking is handled by the Network Manager. In such cases the /etc/sysconfig/SuSEfirewall2 le needs to be edited manually to ensure the FW_ROUTE ag is set to yes : FW_ROUTE="yes" 2.2.1 Networking Limitations on Power Architecture Currently Docker networking has two limitations on the Power architecture. The rst limitation is about iptables. SLE 12 machines cannot run Docker with the iptables support enabled. An update of the kernel is going to solve this issue. In the meantime the Docker package for Power has iptables support disabled via a dedicated directive inside of /etc/ sysconfig/docker . As a result of this limitation Docker containers will not have access to the outer network. A possible workaround is to share the same network namespace between the host and the containers. This however reduces the isolation of the containers. The network namespace of the host can be shared on a per-container basis by adding -net=host to the docker run command. Note: iptables support on SLE 12 SP1 SLE 12 SP1 hosts are not affected by this limitation but, given they use the same SLE 12 package, they will have iptables support disabled. This can be changed by removing the -iptables=false setting inside of /etc/sysconfig/docker . The second limitation is about network isolation between the containers and the host. Currently it is not possible to prevent containers from probing or accessing arbitrary ports of each other. 7 Networking Limitations on Power Architecture SLES 12 SP3 3 Installing sle2docker The sle2docker is used to import pre-built SUSE Linux Enterprise images. The imported prebuilt images can then be used to create base Docker images. The tool is part of the official container module. You can install it by using zypper . But prior to installing sle2docker , verify that the following prerequisites are fulfilled: Ruby is installed on the host machine. The docker daemon is running on the system. The user invoking sle2docker must have proper rights to invoke Docker commands. If the conditions above are fulfilled, you can install the sle2docker tool by running: sudo zypper in sle2docker 8 SLES 12 SP3 4 Storing Images Prior to creating your own images, you should decide where you will store the images. The easiest solution would be to push these images to the Docker Hub (https://hub.docker.com) . By default all images pushed to the Docker Hub are public. This is probably ne as long as this does not violate your company's policy and your images do not contain sensitive data or proprietary software. If you need to restrict access to your Docker images, there are two options: Get a subscription on Docker Hub that unlocks the feature to create private repositories. Run an on-site Docker registry where to store all the Docker images used by your organization or company and combine them with Portus to secure the registry. This chapter describes how to set up an on-site Docker registry and how to combine it with Portus. 4.1 What is a Docker Registry? The Docker registry is an open source project created by Docker Inc. It allows the storage and retrieval of Docker images. By running a local instance of the Docker registry it is possible to completely avoid usage of the Docker Hub. The Docker registry is also used by the Docker Hub. However, the Docker Hub, as seen from the user perspective, is made of the following parts at least: The user interface (UI): The part that is accessed by users with their browser. The UI provides a nice and intuitive way to browse the contents of the Docker Hub either manually or by using a search feature. It also allows to create organizations made by different users. This component is closed source. The authentication component: This is used to protect the images stored inside of the Docker Hub. It validates all push, pull and search requests. This component is closed source. The storage back-end: This is where the Docker images are sent and downloaded from. It is provided by the Docker registry. This component is open source. 9 What is a Docker Registry? SLES 12 SP3 4.2 Installing and Setting Up Docker Registry 1. Install the docker-distribution-registry package: sudo zypper install docker-distribution-registry 2. To automatically start the Docker registry at boot time: sudo systemctl enable registry 3. Start the Docker registry: sudo systemctl start registry The Docker registry configuration is defined inside of /etc/registry/config.yml . With the default configuration the registry listens on ports 5000 and stores the Docker images under /var/lib/docker-registry . Note: Incompatible Versions of Docker and Docker Registry Docker registry version 2.3 is not compatible with Docker versions older than 1.10, because v2 manifests were only introduced with Docker 1.10. As Docker and Docker reg- istry can be installed on different boxes, the versions might be incompatible. If you ex- perience communication errors between Docker and Docker registry, update both to the latest versions. For more details about Docker registry and its configuration, see the official documentation at: https://docs.docker.com/registry/ 10 . Installing and Setting Up Docker Registry SLES 12 SP3 4.3 Limitations The Docker registry has two major limitations: It lacks any form of authentication. That means everybody with access to the Docker reg- istry can push and pull images to it. That also includes the possibility to overwrite already existing images. There is no way to see which images have been pushed to the Docker registry. You can manually take notes of what is being stored inside of it. There is also no search functionality, which makes collaboration harder. The next section is going to introduce Portus, the solution to all of the problems above. 4.4 Portus Portus is an authentication service and user interface for the Docker registry. It is an open source project created by SUSE to address all the limitations faced by the local instances of Docker registry. By combining Portus and Docker registry, it is possible to have a secure and enterprise ready on-premise version of the Docker Hub. Portus is accessible for SLE 12 customers through the Containers module. To install Portus, use the following command: sudo zypper in portus In order to configure Portus properly, follow these steps: 1. First of all, you should install Portus's dependencies if you haven't already. This is thor- oughly documented here: http://port.us.org/docs/setups/1_rpm_packages.html#portus-dependencies . This document will help you to get through the installation process, and it will also warn you about some of the common pitfalls. 2. After installing Portus and its dependencies, you need to configure your instance. The ini- tial setup of Portus is explained here: http://port.us.org/docs/setups/1_rpm_packages.html#initial-setup . When you are done with portusctl , you should modify some config- urable values before using Portus. This is thoroughly explained in this documentation page: http://port.us.org/docs/Configuring-Portus.html . 11 Limitations SLES 12 SP3 3. To apply the configuration changes, restart Apache (this is required after each configura- tion change). 4. Finally, when entering Portus for the rst time, you will be required to enter some infor- mation about your installed registry. For details, see: http://port.us.org/docs/setups/1_rpm_packages#the-default-installation.html . 5. The Portus setup is now complete and you can start using Portus. Currently, Portus is part of SUSE's Docker offer as a technology preview. For more information and documentation about Portus, see: http://port.us.org/ . 12 Portus SLES 12 SP3 5 Creating Custom Images For creating your custom image you need a base docker image of SLES. You can use any of the pre-built SLES images that you can obtain as described in Section 5.2, “Customizing SLES Docker Images”. Note: No SLES Images in Docker Hub Usually you can pull a variety of base docker images from the docker hub (https:// hub.docker.com/) but that does not apply for SLES. Currently we cannot distribute SLES images for Docker because there is no way to associate an End-User License Agreement (EULA) to a Docker image. sle2docker enables you to import pre-built SLES images that you can use for creating base SLES images. After you obtain your base docker image, you can modify the image by using a Dockerfile (usually placed in the build directory). Then use the standard docker building tool to create your custom image: docker build path_to_build_directory For more docker build options please refer to the official Docker documentation (https://docs.docker.com/engine/reference/commandline/build/) . Note: Dockerizing Your Applications You may want to write a dockerfile for your own application that should be run inside a docker container. For a procedure refer to Chapter 6, Dockerizing Applications. 5.1 Obtaining Base SLES Images You can install pre-built images of SLES by using Zypper: sudo zypper in sles11sp4-docker-image suse-sles12sp3-image Pre-built images do not have repositories configured. But when the Docker host has an SLE subscription that provides access to the product used in the image, Zypper will automatically have access to the right repositories. 13 Obtaining Base SLES Images SLES 12 SP3 After the pre-built images are installed, you need to list them using sle2docker to get a proper image name: sle2docker list Now you need to activate the pre-built images: sle2docker activate PRE-BUILT_IMAGE_NAME After successful activation, sle2docker will display the name of the docker image. You can customize the docker image as described in Section 5.2, “Customizing SLES Docker Images”. 5.2 Customizing SLES Docker Images The pre-built images do not have any repository configured and do not include any modules or extensions. They contain a zypper service (https://github.com/SUSE/container-suseconnect) that contacts either the SUSE Customer Center (SCC) or your Subscription Management Tool (SMT) server, according to the configuration of the SLE host that runs the Docker container. The service obtains the list of repositories available for the product used by the Docker image. You can also directly declare extensions in your Dockerfile (for details refer to Section 5.2.3, “Adding SLE Extensions and Modules to Images”. You do not need to add any credentials to the Docker image because the machine credentials are automatically injected into the container by the docker daemon. They are injected inside of the /run/secrets directory. The same applies to the /etc/SUSEConnect le of the host system, which is automatically injected into the /run/secrets directory. Note: Credentials and Security The contents of the /run/secrets directory are never committed to a Docker image, hence there is no risk of your credentials leaking. To obtain the list of repositories use the following command: zypper ref -s It will automatically add all the repositories to your container. For each repository added to the system a new le will be created under /etc/zypp/repos.d . The URLs of these repositories include an access token that automatically expires after 12 hours. To renew the token call the zypper ref -s command. It is secure to commit these les to a Docker image. 14 Customizing SLES Docker Images SLES 12 SP3 If you want to use a different set of credentials, place a custom /etc/zypp/credentials.d/SC- Ccredentials le inside of the Docker image. It contains the machine credentials that have the subscription you want to use. The same applies to the SUSEConnect le: to override the le available on the host system that is running the Docker container, add a custom /etc/ SUSEConnect le inside of the Docker image. Now you can create a custom Docker image by using a Dockerfile . If you want to create a custom SLE 12 image, please refer to Section 5.2.1, “Creating a Custom SLE 12 Image”. If you want to create a custom SLE 11 SP3 Docker image, please refer to Section 5.2.2, “Creating a Custom SLE 11 SP3 Image”. In case you would like to move your application to a Docker container, please refer to Chapter 6, Dockerizing Applications. 5.2.1 Creating a Custom SLE 12 Image The following Docker le creates a simple Docker image based on SLE 12 SP3: FROM suse/sles12sp3:latest RUN zypper ref -s RUN zypper -n in vim When the Docker host machine is registered against an internal SMT server, the Docker image requires the SSL certificate used by SMT: FROM suse/sles12sp3:latest # Import the crt file of our private SMT server ADD http://smt.test.lan/smt.crt /etc/pki/trust/anchors/smt.crt RUN update-ca-certificates RUN zypper ref -s RUN zypper -n in vim 5.2.2 Creating a Custom SLE 11 SP3 Image The following Docker le creates a simple Docker image based on SLE 11 SP4: FROM suse/sles11sp3:latest RUN zypper ref -s 15 Creating a Custom SLE 12 Image SLES 12 SP3 RUN zypper -n in vim When the Docker host machine is registered against an internal SMT server, the Docker image requires the SSL certificate used by SMT: FROM suse/sles11sp4:latest # Import the crt file of our private SMT server ADD http://smt.test.lan/smt.crt /etc/ssl/certs/smt.pem RUN c_rehash /etc/ssl/certs RUN zypper ref -s RUN zypper -n in vim 5.2.3 Adding SLE Extensions and Modules to Images You may have subscriptions to SLE extensions or modules that you would like to use in your custom image. To add them to the Docker image, proceed as follows: PROCEDURE 5.1: ADDING EXTENSION AND MODULES 1. Add the following into your Dockerfile : ADD *.repo /etc/zypp/repos.d/ ADD *.service /etc/zypp/services.d RUN zypper refs && zypper refresh 2. Copy all .service and .repo les that you will use into the directory where you will build the Docker image from the Dockerfile . 16 Adding SLE Extensions and Modules to Images SLES 12 SP3 6 Dockerizing Applications Docker is a technology that can help you to minimize resources used to run or build your appli- cations. There are several types of applications that are suitable to run inside a Docker container like daemons, Web pages or applications that expose ports for communication. You can use Docker for automation of building and deployment processes by adding the build process into a docker image, then building the image and then running containers based on that image. Running your application inside a docker container provides you with the following advantages: You can minimize the runtime environment of the application as you can add to the docker image of the application just the required processes and applications. The image with your application is portable across machines also with different Linux host systems. You can share the image of your application by using a repository. You can use different versions of required packages in the container than the host system uses without having problems with dependencies. You can run several instances of the same application that are completely independent from each other. Using Docker for building of applications provides the following features: You can prepare a complete building image. Your build always runs in the same environment. Your developers can test their code in the same environment as used in production. You can set up an automated building process. The following section provides you with examples and tips on how to dockerize your applica- tions. Prior to reading further, make sure that you have activated your SLES base docker image as described in Section 5.1, “Obtaining Base SLES Images”. 17 SLES 12 SP3 6.1 Running an Application with Specific Package Versions You may face a problem that your application uses a specific version of a package that is different from the package installed on the system that should run your application. You can modify your application to work with another version or you may create a Docker image with that particular package version. The following example of a Dockerfile shows an image based on a current version of SLES but with an older version of the example package FROM suse/sles12sp3:latest MAINTAINER Tux RUN zypper ref && zypper in -f example-1.0.0-0 COPY application.rpm /tmp/ RUN zypper --non-interactive in /tmp/application.rpm ENTRYPOINT ["/etc/bin/application"] CMD ["-i"] Now you can build the image by running in the same directory as the Dockerfile resides: docker build --tag tux_application:latest . The Dockerfile example shown above performs the following operations during the docker build : 1. Updates the SLES repositories. 2. Installs the desired version of the example package. 3. Copies your application package to the image. The source RPM must be placed in the build context. 4. Unpacks your application. 5. The last two steps run your application after a container is started. After a successful build of the tux_application image, you can start a container based on your new image: docker run -it --name application_instance tux_application:latest 18 Running an Application with Specific Package Versions SLES 12 SP3 You have created a container that runs a single instance of your application. Bear in mind that after closing the application, the Docker container exits as well. 6.2 Running Applications with Specific Configuration You may need to run an application that is delivered in a standard package accessible through SLES repositories but you may need to use a different configuration or use specific environment variables. In case you would like to run several instances of the application with non-standard configuration, you can create your own image that will pass the custom configuration to the application. An example with the example application follows: FROM suse/sles12sp3:latest RUN zypper ref && zypper --non-interactive in example ENV BACKUP=/backup RUN mkdir -p $BACKUP COPY configuration_example /etc/example/ ENTRYPOINT ["/etc/bin/example"] The above example Dockerfile results in the following operations: 1. Refreshing of repositories and installation of the example. 2. Sets a BACKUP environment variable (the variable persists to containers started from the image). You can always overwrite the value of the variable with a new one while running the container by specifying a new value. 3. Creates the directory /backup . 4. Copies the configuration_example to the image. 5. Runs the example application. Now you can build the image and after a successful build, you can run a container based on your image. 19 Running Applications with Specific Configuration SLES 12 SP3 6.3 Sharing Data between an Application and the Host System You may run an application that needs to share data between the application's container and the host le system. Docker enables you to do data sharing by using volumes. You can declare a mount point directly in the Dockerfile . But you cannot specify a directory on the host system in the Dockerfile as the directory may not be accessible at the build time. You can nd the mounted directory in the /var/lib/docker/volumes/ directory on the host system. Note: Discarding Changes to the Directory to Be Shared After you declare a mount point by using the VOLUME instruction, all your changes per- formed (by using the RUN instruction) to the directory will be discarded. After the decla- ration, the volume is part of a temporary container that is then removed after a successful build. In case you need to e.g. change permissions, perform the change before you declare the directory as a mount point in the Dockerfile . You can specify a particular mount point on the host system when running a container by using the -v option: docker run -it --name testing -v /home/tux/data:/data sles12sp3:latest / bin/bash Note Using the -v option overwrites the VOLUME instruction if you specify the same mount point in the container. Now let's create an example image with a Web server that will read Web content from the host's le system. The Dockerfile could look as follows: FROM suse/sles12sp3:latest RUN zypper ref && zypper --non-interactive in apache2 COPY apache2 /etc/sysconfig/ 20 Sharing Data between an Application and the Host System SLES 12 SP3 RUN chown -R admin /data EXPOSE 80 VOLUME /data ENTRYPOINT ["apache2ctl"] The example above installs the Apache Web server to the image and copies all your configuration to the image. The data directory will be owned by the admin user and will be used as a mount point to store your web pages. 6.4 Applications Running in the Background Your application may need to run in the background as a daemon or as an application exposing ports for communication. In that case a typical Docker container may be run in background. Do not run your application in the container in the background as it may cause exiting of the container. Run the container in the background instead. An example Dockerfile for an application exposing a port looks as follows: FROM suse/sles12sp3:latest RUN zypper ref && zypper --non-interactive in postfix RUN mkdir -p /var/spool/mail COPY main.cf /etc/postfix/main.cf EXPOSE 25 587 VOLUME ["/var/spool/mail"] ENTRYPOINT ["/usr/sbin/postfix"] Now you can build your image. Docker performs the following operations according to the instructions in the Dockerfile : 1. Docker refreshes repositories and installs the postfix mail server as it is not installed by default in the SLES docker image. 2. The /var/spool/mailboxes directory is created in the le system of the image. The directory will store all mailboxes if you configure the mail server to store data in this directory. 21 Applications Running in the Background SLES 12 SP3 3. Here you copy a configuration le to the particular directory. Make sure that the main.cf is located in the same directory as the Dockerfile . Bear in mind that the configuration will be the same for all instance run in the future. In case you need a different configuration for each container, you will need to edit it after running the container. 4. Each started container will expose the ports: 25 and 587. In case you will run several instances of this image on the same machine, you should define a specific host name for each container. 5. The VOLUME instruction creates a mount point at the /var/spool/mail directory in the container. 6. The last instructions runs the postfix mail server in the started container. After a successful build, you can run a container based on the image: docker run -d --name mail_server -v /var/spool/mail:/var/spool/mail postfix:latest /bin/bash The -d option runs the container in a detached mode and further communication by using the CLI will not be possible. To reattach the container run: docker attach 22 Applications Running in the Background SLES 12 SP3 7 Working with Containers After you have created your images, you can start your containers based on that image. You can run an instance of the image by using the docker run command. The Docker engine then creates and starts the container. The command docker run takes several arguments: A container name - it is recommended to name your container. Specify a user to use in your container. Define a mount point. Specify a particular host name, etc. The container typically exits if its main process finishes. For example, if your container starts a particular application, as soon as you quit the application, the container exits. You can start the container again by running: docker start -ai You may need to remove unused containers, you can achieve this by using: docker rm 7.1 Linking Containers Docker enables you to link containers together which allows for communication between con- tainers on the same host server. If you use the standard networking model, you can link containers by using the --link option when running containers: First create a container to link to: docker run -d --name sles sles12sp3 /bin/bash Then create a container that will link to the sles container: docker run --link sles:sles sles12sp3 /bin/bash The container that links to sles has defined environment variables that enable connecting to the linked container. 23 Linking Containers SLES 12 SP3 A Documentation Updates This chapter lists content changes for this document. This manual was updated on the following dates: Section A.1, “September 2017 (Initial Release of SUSE Linux Enterprise Server 12 SP3)” Section A.2, “November 2016 (Initial Release of SUSE Linux Enterprise Server 12 SP2)” A.1 September 2017 (Initial Release of SUSE Linux Enterprise Server 12 SP3) General Numerous small fixes and additions to the documentation, based on technical feedback. Removed all references to the faillog package, which is no longer shipped (https:// bugzilla.suse.com/show_bug.cgi?id=710788 ). Chapter 1, Docker Overview Added container orchestration description to Section 1.1, “Docker Architecture”. Bugfixes In Section 5.2.3, “Adding SLE Extensions and Modules to Images”, added a proce- dure for adding extensions to custom images (https://bugzilla.suse.com/show_bug.cgi?id=1029270 ). Adjusted names of base images to latest product versions (https://bugzilla.suse.com/show_bug.cgi?id=1050754 ). A.2 November 2016 (Initial Release of SUSE Linux Enterprise Server 12 SP2) General 24 September 2017 (Initial Release of SUSE Linux Enterprise Server 12 SP3) SLES 12 SP3 The e-mail address team@suse.com . for documentation feedback has changed to doc- The documentation for Docker has been enhanced and renamed to Docker Guide. General Changes to this Guide Added Chapter 1, Docker Overview. Updated Chapter 2, Docker Installation (Fate #321136). Added Chapter 6, Dockerizing Applications (Fate #319368). Added Chapter 7, Working with Containers. Bugfixes Rephrased text about restricting access to Docker images (https://bugzilla.suse.com/show_bug.cgi?id=947502 Added configuration for SSL la.suse.com/show_bug.cgi?id=993115 25 ). to ). installation chapter November 2016 (Initial Release of SUSE Linux Enterprise Server 12 SP2) (https://bugzil- SLES 12 SP3 B GNU Licenses This appendix contains the GNU Free Documentation License version 1.2. 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