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
We could - but then eventually we would have to install all possible module dependencies. Which one could argue be OK for the core modules. I would actually be in favour in having them all installed without using options at all - but I fear it would be a nightmare to maintain with the current approach. Adding an option seemed pragmatic at this stage.