For mounting NFS file systems and running nfs-server from within a LXC container on Proxmox 5:
sed -i '$ i\ mount fstype=nfs,\n mount fstype=nfs4,\n mount fstype=nfsd,\n mount fstype=rpc_pipefs,' /etc/apparmor.d/lxc/lxc-default-cgns && systemctl reload apparmor