Turn that frown upside down! If you want to force the decode through NVDEC, do the following:
1. Launch into the plex container shell.
2. Rename "/usr/lib/plexmediaserver/Plex Transcoder" > "/usr/lib/plexmediaserver/Plex Transcoder2"
3. Create a new file called "/usr/lib/plexmediaserver/Plex Transcoder" and put this in the contents:
#!/bin/sh
/usr/lib/plexmediaserver/Plex\ Transcoder2 -hwaccel nvdec "$@"
4. chmod +x "/usr/lib/plexmediaserver/Plex Transcoder"
Done! Your transcodes should be encoded and decoded by nvidia. Plex doesn't reflect it because its a workaround, but check out your CPU utilization, should be negligible.
Unfortunately, you will have to run this everytime the plex docker gets updated until plex adds support for NVDEC officially.