I have the qbittorrent web UI running in a separate container that's attached to my gluetun container. I use the VPN_PORT_FORWARDING_UP_COMMAND option in my gluetun container:
environment:
- VPN_PORT_FORWARDING_UP_COMMAND=/bin/sh -c 'wget -O- --retry-connrefused --post-data "json={\"listen_port\":{{PORTS}}}" http://127.0.0.1:8072/api/v2/app/setPreferences 2>&1'
You may have to adjust the http://127.0.0.1:8072/ part based on your setup.


