Basically you only need port 443 open. Maybe 80 also depending on how you generate the SSL certificates.

Are you able to access (via curl or something else) the port the Lemmy is serving from behind the nginx server?

Well, I'm here again. I get a 504 gateway timeout after upgrading or doing docker-compose down, and found a weird fix for it.

1. docker kill your postgres instance
2. Go to your webpage and wait for it to tell you an error
3. docker-compose up -d and now it'll work

I tried killing postgres and bringing it up immediately as well but that did not fix the problem. Weird magical incantation and downvote me all you want, just wanted to throw it out there.

How's your firewall set up? Some custom firewall rules can end up interfering with Docker's networking, that's worth checking that. It uses iptables to control what can go in and out of the bridge and if the host can access it. If you have a blanket drop policy that overrides Docker's your Docker networking can break.