I know this is very old, but it helped me a bunch (thanks @bombz), so I thought I'd elaborate just a bit.
My plex media is on a different server, a synology NAS - sitting on the same network as my Unraid (which I'm only using for the Plex Server. When setting up Unraid, I added a few remote NFS shares. From the Main panel, they look like this:
Even though these shares have a path on my unraid server (/mnt/remotes/THESDG_dci), my Docker Plex install can't see them until I add these paths to the Plex Docker config:
(and the create/edit view)
Once those were in place, the files were browsable by plex so I could re-link my libraries (I was relocating my plex server from the synology to the unraid).