I just tried using this guide and I couldn’t get it working.
I started with an Nginx proxy to run two sites out of a single Discourse container. I just wanted to add in the error page bit, so I skipped the parts that seemed to overlap with Run other websites on the same machine as Discourse. I must have missed a key step though. In the end I got what I needed from this DigitalOcean tutorial. It’s not hard to manually set this up, but it does seem like there must be a better way.
Given Docker as the standard way to run Discourse, this sounds better. I assume it would be the sort of thing where you’d set it up and forget about it.