9 Creating application images #

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=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"]

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:

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.

To run an instance using a different configuration, create a derived image and include the additional configuration with it. For example, if your application is called example and can be configured using the file /etc/example/configuration_example, you could use:

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:

You can now build the image. After a successful build, you can run a container based on the image you just created.

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 example image contains a Web server that reads Web content from the host's file system. The Dockerfile could look as follows:

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.

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 looks as follows:

FROM registry.suse.com/suse/sle15 1
LABEL maintainer=tux 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"]

Note: Make sure the ports used by the container image are unused

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:

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

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/ .