Summarize and test distDocker.py

Read through tango's distDocker.py ; summarize how it works. It looks like it ssh's into a host and runs the autograder docker images there. Presumably we could test it out by pointing it back to the localhost. This would allow us to avoid Tango's docker in docker. If so, we might also reimplement the localDocker.py so that it avoids docker-in-docker. That could be a big simplification for installation, etc.