Factor apt-get commands out

[fnothaft]

The way we do apt-get update && apt-get install across many containers is kind of wonky and leads to us generating lots of layers that overlap substantially in content but that can't be cached and reused. @cket @jvivian do you all remember why we wrote all the Dockerfiles this way? Refactoring the apt-get update/install process should save a pretty substantial amount of build time while also reducing image size.

[cket]

No, I don't remember. Factoring out a common layer sounds good to me though!

[fnothaft]

Excellent! I'll take a hack at that sometime after #211 merges.