Should we optimize the CircleCI Docker images further?

It's unclear to me whether it's worth the readability tradeoff to inline our shell scripts directly into our `Dockerfile`s… on one hand, having fewer layers will shrink them somewhat, but perhaps it might be sufficient to use something like the new official `--squash` flag (still experimental, I believe), first discussed in [this blog post](https://blog.docker.com/2017/01/whats-new-in-docker-1-13/) from around two years ago.

Whoever performs this task should also poke around the layer diffs to see whether we're missing any low-hanging fruit we can knock off at the same time.