It's set to `custom : br0`, so that I can assign an IP and a dns alias.
EDIT:
After quick digging, 2fauth just spawns nginx inside the docker container to serve the files and the nginx config does not repect the env var.
Nginx config found at a couple of places, but `/etc/nginx/nginx.conf` seems to be the one the container uses. So I just mounted my own config there with the port I want and problem fixed.
Bit hacky, but does the job until it gets fixed upstream.