By default, when using the docker configuration option, the approach used by mesos to run a docker based jenkins slave is by passing the jenkins slave commands as arguments to
the standard shell e.g. /bin/sh -c java -jar ...

There may be cases (for example when wanting to run a docker-in-docker aka
dind image) where a custom shell command is desired instead so that more than just the jenkins slave command can be executed. I.e. start docker then run the jenkins slave command.

This has been highlighted in a google group chat as well. See https://groups.google.com/forum/#!topic/jenkins-mesos/JgFCmqOO7Fg

It would be nice to be able to override this behaviour to make it possible.