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.

Same as for the build, I can imagine that this ``wait`` could take some time. Beside, we probably don't want to wait if ``fast`` is ``True``.