6 Creating Docker Images of Applications #

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 registry.suse.com/suse/sles12sp4: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:

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

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.

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 registry.suse.com/suse/sles12sp4: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:

Now you can build the image and after a successful build, you can run a container based on your image.

You may run an application that needs to share data between the application's container and the host file system. Docker Open Source Engine 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 find 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 performed (by using the RUN instruction) to the directory will be discarded. After the declaration, 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 sles12sp4: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 file system. The Dockerfile could look as follows:

FROM registry.suse.com/suse/sles12sp4:latest

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 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.

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 Open Source Engine container may be run in background. An example Dockerfile for an Apache2 server exposing a port looks as follows:

FROM registry.suse.com/suse/sles12sp4:latest 1
MAINTAINER tux 2
RUN zypper ref -s && zypper --non-interactive in apache2

RUN echo "The Web Server is running" > /srv/www/htdocs/test.html 3
# COPY data/* /srv/www/htdocs/ 4

EXPOSE 80 5

ENTRYPOINT ["/usr/sbin/httpd"]
CMD ["-D", "FOREGROUND"]

Note: Check for Running Apache2 Instances on Your Host

Make sure that you do not have any Apache2 server instances running on your host. The Docker container would not serve any data if you have a host Apache2 server running. Remove or stop any Apache2 servers on your host.

To use the container, proceed as follows:

With the last procedure, you have built an image which was used to test the build process and the Apache2 Web server. If everything was successful, use the following steps to serve your data through the Apache2 Web server:

You can now access your data at http://localhost:80. If you do not want to copy your data into the Docker container, share a specific directory on your host. Refer to https://docs.docker.com/storage/volumes/ for more information.