Add a Docker Latent BuildSlave implementation

This is inspired from https://github.com/elbaschid/dockbot/blob/make_single_command_executable/dockbot/slaves/dockerslave.py.

Apart from the concept few parts are remaining though.

I should have noticed this earlier.  I believe validation of volumes (and generation of `self.volumes`) should be done in constructor, this way any problems would be reported at the config loading stage.