Just putting the Github issue here. More active than these forums.
https://github.com/haugene/docker-transmission-openvpn/issues/1463
Some people have had success changing the PIA login details to the ones you use to log in (p#######) instead of the 'PPTP/L2TP/SOCKS' username and password (x#######). It took like an hour, but this eventually did the trick for me.