I thought the same as you but apparently the current image of qBittorrent on the Asustor App Central store does require https connections on LAN. Trying to disable it bricks the app.
In the meantime, I can’t believe it but I solved my own issue.
In qBittorrent’s Web UI settings I just happened to check “bypass authentication for clients in whitelisted IP subnets” and put in Docker’s default “bridge” network subnet 172.17.0.0/16. Then restarted the container. Lo and behold now it’s connected.
Now if I can just work out how to connect remotely via https, then I can monitor my NAS when away from home.