You should probably add a section on how to remap the ports if need be, port 3000 is pretty common and unlikely to be free on most setups that run more than a handful of containers.

I for sure am not figuring it out since I seem to be stuck in a loop where Jellyfin correctly replies to the login request (at least if its logs are to be believed) and then cliparr boots me back to the source selection screen.

ETA: the issue seems to be with using HTTP instead of HTTPS, quite annoying not to get any error messages to that effect, even though they can be found in the browser console.