Added ianblenke/docker-huginn as docker/ subdirectory

moved Dockerfile out of the root and renamed as Dockerfile.rbenv
On second thought... we don't want to source the .env.example outright, as that would override any environment variables being passed to the docker container.

The ideal thing to do here would be to move this entire thing into the .env.example file itself and just source that.

An alternative thing to do here would be to make the 12factor environment variables passed to the docker container unique somehow. Perhaps prefix each passed variable with DOCKER_, and then default to the non-DOCKER_ prefixed variable name if the DOCKER_ variable name does not exist. That would allow us to source the .env.example and still not collide.