I had some trouble getting nginx to reverse-proxy to transmission using my local IP address. The solution in the end was to add a link from the nginx container to the openvpn container. This causes docker-compose to map the name 'openvpn' to the IP address of the actual container at start up.
This would also allow you to include a portable nginx.conf file in the repository that didn't require the user to change anything. Let me know if you would like me to open a second pull request adding a copy of my nginx.conf that works with this.
Trent Summerfield