You can also use a socks5 server with container:gluetun and run qbt with 127.0.0.1 socks5. (not allowing it to connect to anything but that address in your firewall settings).
By the way you can do this with absolutely any socks5 supporting software (even browsers, ftp clients, etc).
Just set the firewall to allow the software executable to connect only to localhost/127.0.0.1 and you are done.