nixos/hosts/common/optional/fileserver-nfs-mount.nix

{...}:
{
  fileSystems."/mnt" = {
    device = "/dev/vdb";
    fsType = "ext4";
    options = [ ];
  };

 services.nfs.server = {
    enable = true;

    exports = ''
      /mnt/    *(ro,insecure,all_squash)
    '';
  };
}