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.

Yes, please.  Especially with Twisted, it's very important what's in a non-main thread and what is not, as reactors and deferreds can only be used in the main thread.  Typically "thd" has been the signal for something in a non-main thread.