Alright, I did all of the above. I added the proxy_set_header line to Custom Nginx Configuration for the Jellyfin proxy host by hitting the gear in the upper right of the edit window.

The daemon.json file wasn’t on my mini PC (where NPM lives), so I made a new one with those contents sans the 192.168 line. It was already present on my NAS (where Jellyfin lives), but only with a few other lines that weren’t our default-address-pools. I appreciate your previous instructions not assuming that I know the structure of a JSON file, but I do work with them a lot for my day job, so I was comfortable appending to that file on the NAS (though I did have to re-learn some vim basics to do it).

Anyway, after making those daemon.json file changes, I rebooted the whole machine in both places, not just Docker. That should be enough to make any IP addresses refresh themselves, right? But Jellyfin logs still show access coming from 192.168.65.1. I do find it strange that a container would be using an IP address like this one without reporting it somewhere, but in the Inspect tab of Docker Desktop on the mini PC, it just shows that 172.17.0.X address (2 for IPAddress, 1 for Gateway).