Add dockerfile

Add the basic dockerfile.
> In the fabfile and in src/.commonplace

Ok thx @ngokevin  I can introspect the json in the dockerfile.

Longer term maybe we could consider having the global install being a shim like grunt-cli is for grunt. grunt-cli is just a global installed binary that picks up the grunt install local to the tree you're working in. That sort of provides the best of both worlds.