Allow users to specify a Dockerfile to use directly

[magmastonealex]

Team Smurf already uses Docker in day to day development. Building an ENVyfile is an annoying experience for them, because most of their work is transforming the Dockerfile into an ENVyfile - just to have Envy build a docker container.

They suggested that they would like to instead just tell ENVy about their Dockerfile, and move on to defining actions for their project. This allows them to get the best of ENVy's environment management, without re-writing files they've already made.