Let me describe my use case more precisely.
Currently I have a VM with Rockstor (a NAS system) which also have rock-ons (simply addons) and in this case it have Plex Media Server (or PMS) as an addon. Rockstor addons are docker containers based on official documentation. PMS transcoding can be pretty demanding sometimes so I want to optimize and reorganize it, by moving PMS to its own proxmox container but it still need to have access to certain folders of interest located on Rockstor within the VM to pull all the media from. Network share would be the easiest way I agree, but the question is, can I make it more efficient - can I access folder of interest from the VM directly without network share simply as local folder? Rockstor have its own disk organized as pools but actually data in them is pretty much a subfolder somewhere on a btrfs sysm. Maybe defined as subvolume. I wonder if VM's disk is just a folder somewhere on Proxmox, or there are tools to access VM's disk image more efficiently than a network share between VM and container.