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.

This functionality was directly taken from the original work. I did not tried it (hence no tests), it probably works, but I don't really know how to describe it ...

We could remove it in a first time, and re-introduce it if asked ...