added option to install with support for the docker module

added docker-py dependency (used by docker modules that is now part of the core)
activate docker-py resource
docker option
If a list exists or can be generated for all of the possible dependencies of the core modules, that would be great; I think installing everything is the better solution.

I don't see how adding a --with-docker option makes this formula easier to maintain; can you clarify? I would prefer that this option is removed before pulling this.