Why can't I use Docker CMD multiple times to run multiple services?

Solution 1:

You are right, the second Dockerfile will overwrite the CMD command of the first one. Docker will always run a single command, not more. So at the end of your Dockerfile, you can specify one command to run. Not more.

But you can execute both commands in one line:

FROM centos+ssh
EXPOSE 22
EXPOSE 4149
CMD service sshd start && /opt/mq/sbin/rabbitmq-server start

What you could also do to make your Dockerfile a little bit cleaner, you could put your CMD commands to an extra file:

FROM centos+ssh
EXPOSE 22
EXPOSE 4149
CMD sh /home/centos/all_your_commands.sh

And a file like this:

service sshd start &
/opt/mq/sbin/rabbitmq-server start

Solution 2:

Even though CMD is written down in the Dockerfile, it really is runtime information. Just like EXPOSE, but contrary to e.g. RUN and ADD. By this, I mean that you can override it later, in an extending Dockerfile, or simple in your run command, which is what you are experiencing. At all times, there can be only one CMD.

If you want to run multiple services, I indeed would use supervisor. You can make a supervisor configuration file for each service, ADD these in a directory, and run the supervisor with supervisord -c /etc/supervisor to point to a supervisor configuration file which loads all your services and looks like

[supervisord]
nodaemon=true

[include]
files = /etc/supervisor/conf.d/*.conf

If you would like more details, I wrote a blog on this subject here: http://blog.trifork.com/2014/03/11/using-supervisor-with-docker-to-manage-processes-supporting-image-inheritance/