Mount volume in cache to persist issued certificates

[kdhageman]

When running the cache over TLS, the certmagic lib automatically obtains certificates. The issued certificates are stored on disk, but because we currently do not mount a volume to persist those certificates, they disappear whenever the container is closed, and with a restart a new cert is issued. As a result, the rate limit of Let's Encrypt may be hit, locking us out of running on TLS for a few days.

The docker-compose config should mount a volume to the correct location where the certs are stored by certmagic