You can't use an nfs:// address as a location map, docker does not recognize that. You need to mount the nfs share in libreelec to a local path, and use that local path in addon settings
Thank you for the hint. That did the trick - had to google a bit, how to mount a nsf shared folder, but I got it to worked now. Great Addon 
Hey there,
I just installed Plex via the LE docker repo. I have access to the server and it looks like its running so far.
But I can't add any share media folders from my NAS. I tried to Map the NSF Share in the settings, as you described, but when I do this I can't access Plex anymore (it just wont't start). I disabled and enabled the Addon, restarted Kodi (using Libreelec 8.2.5. with RP2), but nothing helps. If i chance the mapping back to the standard "/storage/tvshows" (for example) Plex is starting again.
The NSF share is working fine, since I am using it for my Kodi library. I also tried SMB share but with the same issue.
May I miss something!?
edit:
Some more information. When I edit the settings file to map to my shared network folder, it looks like the whole image isn't even recognized by Docker.
I tried to use the commnd "docker ps -a" to to see all images, but its completley blank. When I reset the settings to default the image show perfectly and Plex is starting.
I don't think I did anything wrong with the mapping since I am using the tree folder structure inside Kodi to point to the right direction. So I can't see why Docker can't recognize the image after I changed the settings?
Here is my changed setting file:
<settings>
<setting id="E_VERSION" value="public" />
<setting id="V_config" value="/storage/.kodi/userdata/addon_data/docker.linuxserver.plex" />
<setting id="V_data_movies" value="nfs://192.168.178.100/Media/Movies/" />
<setting id="V_data_tvshows" value="/storage/tvshows" />
<setting id="V_transcode" value="/storage/downloads/plextemp" />
</settings>