Edit: This post has been made obsolete in recent releases, which allow enabling these flags via both the web UI and the pct CLI tool. Normally, Proxmox doesn’t allow mounting NFS mounts directly in containers due to security concerns. In the past it was possible to modify apparmor profiles directly in order to allow it, as seen here and here. However, as of this commit, that method is no longer an option, due to the apparmor profiles now being generated dynamically when the containers are started (you can view these profiles at /var/lib/lxc/${CID}/apparmor/lxc-${CID}_<-var-lib-lxc>).