Docker LMS running on Synology can't save playlist - permissions question

Not exactly sure where to post this… apologies if my aim is off.
Running Logitech Media Server 8.2.1 via Docker on a Synology DSM 7.x box.
All works fine, but although it can find playlists it can’t seem to save them or update one after loading/removing a song. No errors other than this within LMS:[22-09-12 08:40:40.4446] Slim::Formats::Playlists::Base::_filehandleFromNameOrString (86) Error: Could’t open /playlist/60’s.m3u for writing.

I realize it’s a permissions issue, I just don’t know where/how to fix.
Within Docker the PUID is assigned to an admin account. PGID 100. The playlists volume mount path is rw.
Within DSM shared folder - Permmision for the mapped user is rw, the admin group as well, http has no access and users are RO. There are no NFS permissions set for the volume.
I’m missing something, but no idea what. Any nudge in the right direction much appreciated!