nixos/containers: add extraFlags option

to pass extra flags to systemd-nspawn

nixos/modules/virtualisation/containers.nix

@@ -575,6 +575,16 @@ in
               '';
             };
 
+            extraFlags = mkOption {
+              type = types.listOf types.str;
+              default = [];
+              example = [ "--drop-capability=CAP_SYS_CHROOT" ];
+              description = ''
+                Extra flags passed to the systemd-nspawn command.
+                See systemd-nspawn(1) for details.
+              '';
+            };
+
           } // networkOptions;
 
           config = mkMerge
@@ -714,7 +724,9 @@ in
             ${optionalString cfg.autoStart ''
               AUTO_START=1
             ''}
-            EXTRA_NSPAWN_FLAGS="${mkBindFlags cfg.bindMounts}"
+            EXTRA_NSPAWN_FLAGS="${mkBindFlags cfg.bindMounts +
+              optionalString (cfg.extraFlags != [])
+                (" " + concatStringsSep " " cfg.extraFlags)}"
           '';
       }) config.containers;