There is no way for a property of a docker container to require
some property be met outside the container. For example, some servers
need ntp installed for a good date source.
propellor does not build docker containers, I think that's the point where a CMD is set.
It would probably make sense to have a mode where docker run is not passed any explicit command to run, which would let the predefined CMD be used. Although this would not let propellor run inside the container, so it could not perform any provisioning of it. In this mode, propellor would only be able to ensure that a container was installed and start it running with its default configuration.
It would be great to be able to set the CMD of a docker container.
http://docs.docker.io/reference/builder/#cmd
propellor does not build docker containers, I think that's the point where a CMD is set.
It would probably make sense to have a mode where docker run is not passed any explicit command to run, which would let the predefined CMD be used. Although this would not let propellor run inside the container, so it could not perform any provisioning of it. In this mode, propellor would only be able to ensure that a container was installed and start it running with its default configuration.