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.

As I mentioned at that time, this code is taken verbatim from upstream, I did not checked it neither tested it, hence my original question about leaving it or re-implementing it when needed ?