edit: a working solution is proposed by @Lifebandit666@feddit.uk below:

So you’re trying to get 2 instances of qbt behind the same Gluetun vpn container?

I don’t use Qbt but I certainly have done in the past. Am I correct in remembering that in the gui you can change the port?

If so, maybe what you could do is set up your stack with 1 instance in, go into the GUI and change the port on the service to 8000 or 8081 or whatever.

Map that port in your Gluetun config and leave the default port open for QBT, and add a second instance to the stack with a different name and addresses for the config files.

Restart the stack and have 2 instances.

Has anyone run into issues with docker port collisions when trying to run images behind a bridge network (i think I got those terms right?)?

I'm trying to run the arr stack behind a VPN container (gluetun for those familiar), and I would really like to duplicate a container image within the stack (e.g. a separate download client for different types of downloads). As soon as I set the network_mode to 'service' or 'container', i lose the ability to set the public/internal port of the service, which means any image that doesn't allow setting ports from an environment variable is stuck with whatever the default port is within the application.

Here's an example .yml:

services:
  gluetun:
    image: qmcgaw/gluetun:latest
    container_name: gluetun
    cap_add:
      - NET_ADMIN
    environment:
      - VPN_SERVICE_PROVIDER=mullvad
      - VPN_TYPE=[redacted]
      - WIREGUARD_PRIVATE_KEY=[redacted]
      - WIREGUARD_ADDRESSES=[redacted]
      - SERVER_COUNTRIES=[redacted]
    ports:
      - "8080:8080" #qbittorrent
      - "6881:6881"
      - "6881:6881/udp"
      - "9696:9696" # Prowlarr
      - "7878:7878" # Radar
      - "8686:8686" # Lidarr
      - "8989:8989" # Sonarr
    restart: always

  qbittorrent:
    image: lscr.io/linuxserver/qbittorrent:latest
    container_name: "qbittorrent"
    network_mode: "service:gluetun"
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=CST/CDT
      - WEBUI_PORT=8080
    volumes:
      - /docker/appdata/qbittorrent:/config
      - /media/nas_share/data:/data)

Declaring ports in the qbittorrent service raises an error saying you cannot set ports when using the service network mode. Linuxserver.io has a WEBUI_PORT environment variable, but using it without also setting the service ports breaks it (their documentation says this is due to CSRF issues and port mapping, but then why even include it as a variable?)

The only workaround i can think of is doing a local build of the image that needs duplication to allow ports to be configured from the e variables, OR run duplicate gluetun containers for each client which seems dumb and not at all worthwhile.

Has anyone dealt with this before?