Here is also the output of mtab in the container
proxmox:/var/lib/vz/private/104/etc# cat mtab
none /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
none /proc proc rw,nosuid,nodev,noexec,relatime 0 0
udev /dev tmpfs rw,relatime,size=10240k,mode=755 0 0
/dev/mapper/pve-root / ext3...