To build on this since I have this setup now, it basically creates a new docker network that you can attach containers to, and have all of their traffic routed through it. Basically I have the gluetun container running, then in my qbittorrent docked-compose I have network_mode: "container:gluetun".

One thing to watch out for is you have to specify the ports in the gluetun docked-compose instead of in each docked-compose.

Additionally, if gluetun shuts down and the apps using it don't, you'll have to restart the apps using it. Not an issue if it's all in the same docker-compose file, but I like separating docker-compose services so I have qbittorrent/docker-compose.yml and gluetun/docker-compose.yml