Jellyfin image does not respect PUID and PGID

[NotSaifA]

The official Jellyfin image does not respect the PUID and GUID environment variables and runs as root, this can be solved using 2 ways.

1. adding user: "$PUID:$PGID" to the service declaration.
2. switching to the linuxserver/jellyfin image.

I didn't make a pull request because changing the permissions from root to the user and docker group messes with existing setups because ownership has to be changed manually for existing files.