Automatically detect port from env

[5HT2]

Currently, you must edit both config/.env and docker-compose.yml to change the port of a Docker container. This is confusing to the user (and to me, when I forget about it).

Grabbing the port from config/.env would be ideal.

Currently we have HB_ADDR=0.0.0.0:6060, which is part of what we need. We could make the 6060 part a HB_PORT variable in config/.env if Docker supports expanding env vars.

Testing it needed to ensure if this is possible.

[5HT2]

I did 2f93848, which doesn't entirely solve things (two variables inside config/.env), but at least you only have to edit one file now.