Added ianblenke/docker-huginn as docker/ subdirectory

moved Dockerfile out of the root and renamed as Dockerfile.rbenv
Would it make sense to have it read the current defaults from .env.example, like https://github.com/evaryont/huginn-chef/commit/0de1905b554a77acd33b145947b2aa885835dd75#commitcomment-7775647 ?