Implement persistent state across docker runs

[alexcrichton]

The prod script for rust-central-station right now notably mounts a few volumes for persisting data across runs of the container.

This is useful for information like lets encrypt certificates, logs, secrets, etc. Right now rust-central-station also accesses some config files at runtime instead of at container build time, but we probably shouldn't support that.

[aidanhs]

We're down to just secrets and logs now. Secrets are covered by #2, I'm fine with logs going to disk for now, but at some point we'll probably want a better solution (be that moving to heroku, using a docker logs driver or running a program to ship the logs off to some aws service).