How did you run up the docker instance?
Docker run or compose?
What were your settings?
A good thing about docker is that you can host ports to whatever you want as the internal docker ports are still the same.
In docker run it’s the -p flag, so [host]:443 -p [host2]:4443 or in docker compose
ports
- host:443
Then with an nginx reverse proxy all you have to do is point it to the right ports. However, there are a bunch of nginx flags/settings to make something like nextcloud work properly. What’s your nginx configuration?
I suspect it’s got something to do with this setting X-Frame-Options: SAMEORIGIN but I’m not expert so
I suspect this is where the issue lies but it might be some other Synology related thing.