Standardize "apt-get install" usage across the repo

I might have missed some, but I think this is most of the offenders.

Of course, this includes a cache-bust just for you @tiborvass. :)
Not necessarily - this file (https://get.docker.io/ubuntu/) is supposed to be more documentary than actual runtime code, so I think we should actually drop the `-y` here to match the rest of the places in the code where we do interactive `apt-get install`, but it's definitely open for discussion.