Combine commands when applicable

Thanks for being awesome about keeping this up to date! I noticed during a fresh build the other day that there are a *lot* of layers coming out of this base image, so this is a PR to help clean that up a little. All I've done is combine run commands when applicable, which should have a few side effects.

- Meaningful build cache
- Fewer FS layers

All in all, this takes a build from 32 steps to ~~18~~ 16 while still staying fairly readable (imo). It also decreases the image size by 79.6mb (which isn't massive, but at least it's a little something).

Update: By using `apt-key adv --fetch-keys`, we don't need wget and multiple apt-get update / installs. Also by moving the cleanup to the layer that apt-get actually runs in, we're able to actually clean things up *in that layer*, which is what we really want to do.