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.

We might have this need now that you are working on TRAC:3048: minimising a number of error situations might help to deal w/ the problem as well (and, yes, I understand that the traceback happens later).