plex linux nvidia hw decode was released on beta @ Plex Media Server 1.17.0.1709
if your server is newer than 1.17.0.1709, then HW should be available.
dockers complicate things, perhaps you should try plex on baremetal, confirm it does or does not work there first.
otherwise, I would suggest you either open a new thread or edit the OP title and provide a lot more details about your hardware, os, docker config, and including logs from pms reproducing the problem.
