Hello,
For remember : https://forum.proxmox.com/threads/proxmox4-lxc-bind-mounts-read-only.25995/
The option "ro" on lxc mount it's ready, now update the documentation (https://pve.proxmox.com/wiki/LXC_Bind_Mounts) for support option (ro=1)
Exemple : mp0:/host-target,mp=/guest-target,ro=1
Thanx