i know this post is a bit old but there doesnt seem to be much info out there on it. May be able to help someone else looking. i finally got this to work.... once plex, nvidia-docker and the nvidia driver are installed correctly like mentioned in the first post '--runtime=nvidia' can be added in portainer. if you edit the plex container in portainer... go to the runtime & resources tab at the bottom and 'nvidia' will appear in the runtime pull down menu. you also need to add 2 environment variables:
NVIDIA_DRIVER_CAPABILITIES = compute,video,utility
and
NVIDIA_VISIBLE_DEVICES = your GPU UUID
your GPU UUID can be found by typing:
nvidia-smi -q
in the command line and your GPU UUID will be near the top of the list
