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 (``build``) can take quite some time, especially when pulling the base image from internet ... It is a generator that spew the logs as it happens. Any way we could put that in a deferred ? Does it matters at all ?