Applies to Container Guide

10 Creating application images #

Docker Open Source Engine is designed to allow running multiple separate application environments in parallel, with lower resource use than when using full virtual machines. Applications that are suitable for running inside containers include daemons, Web servers, and applications that expose IP ports for communications. You can use Docker Open Source Engine to automate the building and deployment processes by performing the build process inside a container, building an image, and then deploying containers based on the image.

Running an application inside a container has the following advantages:

The image with the application is portable across servers running different Linux host distributions and versions.
You can share the image of the application using a repository.
You can use different versions of software in the container and on the host system, without creating dependency issues.
You can run multiple instances of the same application that are independent from each other.

Using Docker Open Source Engine to build applications has the following advantages:

You can prepare an image of the complete build environment.
The application can run in the same environment it was built in.
Developers can test their code in the same environment as used in production.

The following section provides examples and recommendations on creating container images for applications. Before proceeding, make sure that you have activated your SUSE Linux Enterprise Server base image as described in Section 8.1, “Pulling base SLES images”.

10.1 Running an application with specific package versions #

If your application needs a version of a package different from the package installed on the system, you can create a container image that includes the package version the application requires. The following example Dockerfile allows building an image based on an up-to-date version of SUSE Linux Enterprise Server with an older version of the example package:

FROM registry.suse.com/suse/sle15
LABEL maintainer=EXAMPLEUSER_PLAIN
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"]

Build the image by running the following command in the directory that the Dockerfile resides in:

> docker build --tag tux_application:latest .

The Dockerfile example shown above performs the following operations during the docker build:

Updates the SUSE Linux Enterprise Server repositories.
Installs the desired version of the example package.
Copies the application package to the image. The binary RPM must be placed in the build context.
Unpacks the application.
The last two steps run the application after a container is started.

After a successful build of the tux_application image, you can start a container based on the new image using the following command:

> docker run -it --name application_instance tux_application:latest

Keep in mind that after closing the application, the container exits as well.

10.2 Running an application with a specific configuration #

To run an instance using a different configuration, create a derived image and include the additional configuration with it. In the example below, an application called example is configured using the file /etc/example/configuration_example:

FROM registry.suse.com/suse/sle15 1
RUN zypper ref && zypper --non-interactive in example 2
ENV BACKUP=/backup 3
RUN mkdir -p $BACKUP 4
COPY configuration_example /etc/example/ 5
ENTRYPOINT ["/etc/bin/example"] 6

The above example Dockerfile performs the following operations:

1	Pulls the `sle15` base image as described in Section 8.1, “Pulling base SLES images”.
2	Refreshes repositories and installations of the example.
3	Sets a `BACKUP` environment variable (the variable persists to containers started from the image). You can always overwrite the value of the variable while running the container by specifying a new value.
4	Creates the directory `/backup`.
5	Copies the `configuration_example` to the image.
6	Runs the `example` application.

10.3 Sharing data between an application and the host system #

Docker Open Source Engine allows sharing data between host and a container by using volumes. You can specify a mount point directly in the Dockerfile. However, you cannot specify a directory on the host system in the Dockerfile, as the directory may not be accessible at build time. Find the mounted directory under /var/lib/docker/volumes/ on the host system.

Note: Discarding changes to the directory to be shared

After you specify a mount point by using the VOLUME instruction, all changes made to the directory with the RUN instruction are discarded. After the mount point is specified, the volume becomes a part of a temporary container, which is removed after a successful build. This means that for certain actions to take effect, they must be performed before specifying a mount point. For example, if you need to change permissions, do this before you specify the directory as a mount point in the Dockerfile.

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 sles12sp4:latest /bin/bash

Note

The -v option overwrites the VOLUME instruction if you specify the same mount point in the container.

The following Dockerfile example builds an image containing a Web server that reads Web content from the host’s file system:

FROM registry.suse.com/suse/sles12sp4
RUN zypper ref && zypper --non-interactive in apache2
COPY apache2 /etc/sysconfig/
RUN chown -R admin /data
EXPOSE 80
VOLUME /data
ENTRYPOINT ["apache2ctl"]

The example above installs the Apache Web server to the image and copies the entire configuration to the image. The data directory is owned by the admin user and is used as a mount point to store Web pages.

10.4 Applications running in the background #

If your application needs to run in the background as a daemon, or as an application exposing ports for communication, you can run the container in the background.

An example Dockerfile for an application exposing a port is as follows:

FROM registry.suse.com/suse/sle15 1
LABEL maintainer=EXAMPLEUSER_PLAIN 2
ADD etc/ /etc/zypp/ 3
RUN zypper refs && zypper refresh 4
RUN zypper --non-interactive in apache2 5
RUN echo "The Web server is running" > /srv/www/htdocs/test.html 6
# COPY data/* /srv/www/htdocs/ 7
EXPOSE 80 8
ENTRYPOINT ["/usr/sbin/httpd"]
CMD ["-D", "FOREGROUND"]

1	Pull the base image as described in Section 8.1, “Pulling base SLES images”.
2	Maintainer of the image (optional).
3	The repositories and service files to be copied to `/etc/zypp/repos.d` and `/etc/zypp/services.d`. This makes them available on the host in the container.
4	Command to refresh repositories and services.
5	Command to install Apache2.
6	Test line for debugging purposes. This line can be removed if everything works as expected.
7	A `COPY` instruction to copy data from the host system to the directory in the container used by the server. The leading hash character ( `#` ) marks this line as a comment; it is not executed.
8	The exposed port for the Apache Web server.

Note

To use port 80, make sure there is no other server software running on this port on the host.

To use the container, proceed as follows:

Prepare the host system for the build process.
1. Make sure the host system is subscribed to the Server Applications Module of SUSE Linux Enterprise Server. To view installed modules or install additional modules, open YaST and select Add System Extensions or Modules.
2. Make sure the SLE images from the SUSE Registry are installed as described in Section 8.1, “Pulling base SLES images”.
3. Save the Dockerfile in the docker directory.
4. Within the container, you need access to software repositories and services that are registered on the host. To make them available, copy repositories and service files from the host to the docker/etc directory:
```
> cd docker
> mkdir etc
> sudo cp -a /etc/zypp/{repos.d,services.d} etc/
```
  Instead of copying all repository and service files, you can also copy only the subset that is required by the container.
5. Add Web site data (such as HTML files) into the docker/data directory. The contents of this directory are copied to the container image and are thus published by the Web server.
Build the container. Set a tag for your image with the -t option (in the command below, it is EXAMPLEUSER_PLAIN/apache2):
```
> docker build -t EXAMPLEUSER_PLAIN/apache2 .
```
Docker Open Source Engine executes the instructions provided in the Dockerfile: pull the base image, copy content, refresh repositories, install the Apache2, etc.
Start a container instance from the image created in the previous step:
```
> docker run --detach --interactive --tty EXAMPLEUSER_PLAIN/apache2
```
Docker Open Source Engine returns the container ID, for example:
```
7bd674eb196d330d50f8a3cfc2bc61a243a4a535390767250b11a7886134ab93
```
Point a browser to http://localhost:80/test.html . You should see the message The Web server is running.

To see an overview of running containers, run the docker ps --latest command:

> docker ps --latest
CONTAINER ID        IMAGE               COMMAND                  [...]
7bd674eb196d
tux/apache2         "/usr/sbin/httpd -..."   [...]

To stop and delete the container, run the following command:

> docker rm --force 7bd674eb196d

You can use the resulting container to serve your data with the Apache2 Web server by following these steps:

In the Dockerfile :
- In the example Dockerfile, comment the line that starts with RUN echo by adding a # character at its beginning.
- In the example Dockerfile, uncomment the line starting with COPY by removing the leading # character.
Rebuild the image.

Run the image in detached mode:

> docker run --detach --interactive --tty EXAMPLEUSER_PLAIN/apache2

Docker Open Source Engine responds with the container ID, for example:

e43fff4ae9832ecdb7677c058a73039d7610c32145a1d9b6ad0a4ed52b5c4dc7

To view the published data, point a browser at http://localhost:80/test.html .

To avoid copying Web site data into the container, share a directory of the host with the container. For more information, see https://docs.docker.com/storage/volumes/ .