Yes, indeed you overwrote my post, you have a log of privileges 
But I could get my text from the history like you wrote. That was:
I have a CT via Proxmox Helper Scripts | Proxmox Scripts For Home Automation. Nevertheless, I think i would also get my HDD in there as shown in your screenshot, have not tested that but read how to do this.
But to @theCalcaholic as said the current datadir of my installation is /var/www/nextcloud/data. From what I meanwhile read I would need to mount the drive to any mount point that I choose (via console input), and then edit the config.php manually to point to that datadir, as for some reason that option is not available in ncp-config?
I have an existing HDD from my current RPi installation in BTRFS format that hosts the datadir as well das the NCP database.
The database I do not need, the HDD I would in the simplest case like to continue to use in the new proxmox installation. In the worst case I would reinitialize the HDD and copy the data then back from a backup.
BTRFS in Proxmox also seems to be not the easiest thing from what I read and also not possible from the GUI: [SOLVED] - BTRFS mount point in LXC | Proxmox Support Forum
So if it is easier I might also go with EXT4 instead.
As I see in your screenshot that you also added BTRFS I would indeed be interested in your how-to 