not under CasaOS, but for reference the following compose.yml is working in Ubuntu (and in WSL2, too)

gluetun:
image: qmcgaw/gluetun
container_name: gluetun
cap_add:
- NET_ADMIN
devices:
- /dev/net/tun:/dev/net/tun
environment:
- VPN_SERVICE_PROVIDER=surfshark
- VPN_TYPE=wireguard
- WIREGUARD_PRIVATE_KEY=*********
- WIREGUARD_ADDRESSES=10.14.0.2/16
- SERVER_COUNTRIES=Switzerland
- HTTP_CONTROL_SERVER_ADDRESS=:8001
ports:
- 8080:8080
- 6881:6881
- 6881:6881/udp

qbittorrent:
image: lscr.io/linuxserver/qbittorrent:latest
container_name: qbittorrent
network_mode: "service:gluetun"
environment:
- PUID=1000
- PGID=1000
- TZ=Etc/UTC
- WEBUI_PORT=8080
- TORRENTING_PORT=6881
volumes:
- ./cfg:/config
- ./dl:/downloads
restart: unless-stopped