...a group 'media-group' (with gid 1200), and added root to this group
usermod -aG media-group root
on proxmox host, added a mountpoint and idmap for 'media-group' to the config file /etc/pve/lxc/[lxc_id].conf
mp0: /mnt/truenas-media,mp=/mnt/media,backup=0
lxc.idmap: u 0 100000 65536
lxc.idmap...