If we're actually talking proxies: If you enable UDP-based protocols, it's very hard to avoid IP leaking. A surprising amount of clients just don't support proxying UDP at at all, or misbehave in various ways.
Word of advice: Set up the torrent client in a dedicated VM (or box) and set it up on kernel-level to route all its traffic through a separate VM(/box), which itself connects to the VPN (Wireguard,OVPN or what-have-you) and forwards traffic. It sounds complex but is robust and avoids a lot of potential pitfalls.
Reliably routing P2P UDP traffic with container networks is a fool's errand so I wouldn't recommend Docker networks (ofc fine to run the container with network=host tho)