New Docker user - "Trusted domain" problems

Been using NC for a while on Synology

Unfortunately the changes in NC21 broke my setup (php version hell). So I’m trying to go over to running NC in Docker on Synology in the hope of having a setup that is decoupled from the php stuff delivered by Synology.

This is proving much more difficult than I expected.

I have been able to create a NC instance (+mariaDB) with the recommended image.
When I try to access NC for initial setup at
I get the dreaded " Access through untrusted domain".

I’m using OpenVPN on my pfSense firewall, so I have no need to configure access from the outside world (as far as NC instance knows).

I have tried a lot of internet searching, but am confused where and which “magic words” are needed to tell my NC docker instance what domains are trusted.

I’ve dabbled with Linux for 20 years but as a HW engineer, not SW, my knowledge and time to learn intricacies are unfortunately limited, already spent time setting up OpenVPN on my custom pfSense box + lots of time getting NC to work directly on Synology - which is now somewhat wasted since trying to go over to Docker.

Edit: I added
localhost 172.30.30.* 10.175.10.*

to docker environment tab in Synology Docker configuration.
This does not work for my existing nextcloud docker instance - but does work if I create a new docker instance with this env variable.
How to get this to work on my existing docker instance?

I’m having a similar issue in that I can’t edit the config.php file to include additional Trusted Domains without borking my Nextcloud instance and having to redeploy the entire stack. Ever figure out your troubles?