Postgres example using docker

@rgbkrk had mentioned interest in a Postgres Dockerfile for jupyterhub.  This is roughly what I've been using for local development for the past few days.
It's actually not _that_ bad for people to use the default if they're linking containers, since there are no ports on the DB exposed to the outside world.  But if they're running it with host ports open then they should definitely change the defaults.  Maybe we could throw an angry warning if the password is arglebargle?