Many various Dockerfile cleanups

Update Dockerfile to use stackbrew/ubuntu (until it graduates), update Dockerfile MAINTAINER line, coalesce all apt-get installs into one invocation (including s3cmd by bringing in backports).

By having all the apt-get stuff on one line, we get automatic cache busting when necessary, and cleaner/proper retry if `apt-get update` somehow fails (see #3342).

I've also slimmed down our image slightly by using `--no-install-recommends`, and making sure we have all our explicit deps (as far as I can tell in my own testing - if we're missing any, please holler of course!).