I just setup my Plex server using docker and at the beginning I used my standard user user1 as it is shown in the guide video. Plex just went well. Everything is up to date, media files were found and meta data loaded. 
But: Some guys recommend a dedicated docker user for better permission management. So let's try it out! Added user 'dockerUser', assigned group users and changed PUID in plex container. Now I'm having that issue that my media library seems to be empty. I can change configuration stuff (So access to /config seems to be ok I guess) and I can add a library (where I have to browse to the media folder, so access to /media seems to be ok too) but Plex somehow cant find the media files itself? When I try to browse a library its just saying "Library is empty".
Are there any system groups dockerUser has to be assigned to or am I missing sth. else? 
And by the way: I dont really get the sense of PGID option. In the guide, id of 'users' is used. Does that mean the docker container can only get permissions of that one group? What about groups PUID user is assigned to anyway?
Thank you!