Changes init-plex-chown/run to use TMPDIR instead of hardcoded /run/plex-temp
Plex uses TMPDIR to store transcodes of music to WAV format for music
analysis (and maybe other things).
LS.io container sets TMPDIR=/run/plex-temp. With read-only root this
means the /run tmpfs needs to have enough memory to store music analysis
transcodes.
LS.io container hardcoded /run/plex-temp in init so TMPDIR could not be
flexibly changed -- changed to use TMPDIR and clean on restart in case
of unclean shutdown.
Plex container is marked as supporting read-only root.
