RFC: Docker-based development support

Here's a first pass at supporting a 100% Docker-based development workflow. See `HACKING.md` for details, rationale, etc.

TODO:
[ ] More extensive testing (also: figure out what's up with the guestbook example)
[ ] Break up the cmd refactor into a separate commit

Possibly optional TODO:
[ ] Add cleanup for k8s containers
[ ] Make minions with a generator to support N minions
[ ] More refactoring of cmd (took a crack at method extraction but didn't refactor input handling and duplication)