Docker needs to support running a user specified command in an existing container. This feature was discussed during the docker-dev contributors meeting on 6/19/14 -
https://botbot.me/freenode/docker-dev/2014-06-19/?page=4. @tianon had mentioned that this is a feature that has been pending for a while.
Both lxc and our native driver has support for this feature already.