I am also having the issue with with the docker.img getting maxed out on size with the linuxserver plex container. I haven't changed anything other than just doing the standard update container option in the unraid gui. The problem for me started around the time Plex enabled the new skip intro feature. The tmp folder is what is eating up all the space, in about 2-3 days it ballooned to 27 GB.
Docker run command is as follows although nothing has changed on my side in months in regards to config but the problem started about 2 weeks ago. "docker run -d --name='Plex' --net='bond0' --ip='10.0.1.230' -e TZ="America/New_York" -e HOST_OS="Unraid" -e 'NVIDIA_DRIVER_CAPABILITIES'='all' -e 'NVIDIA_VISIBLE_DEVICES'='GPU-163441c5-a503-1b9f-b353-1b36622ff622' -e 'PUID'='99' -e 'PGID'='100' -e 'CHANGE_CONFIG_DIR_OWNERSHIP'='true' -e 'VERSION'='latest' -v '/mnt/user/Containers/Plex/Data/':'/config':'rw' -v '/dev/shm/':'/transcode':'rw' -v '/mnt/user/Plex/':'/data':'rw' --runtime=nvidia --log-opt max-size=50m --log-opt max-file=1 'linuxserver/plex:latest'"