nixos/nixos-containers: add specialArgs option (#216677)

nixos/modules/virtualisation/nixos-containers.nix

@@ -514,6 +514,7 @@ in
                       };
                     in [ extraConfig ] ++ (map (x: x.value) defs);
                   prefix = [ "containers" name ];
+                  inherit (config) specialArgs;
                 }).config;
               };
             };
@@ -555,6 +556,16 @@ in
               '';
             };
 
+            specialArgs = mkOption {
+              type = types.attrsOf types.unspecified;
+              default = {};
+              description = lib.mdDoc ''
+                A set of special arguments to be passed to NixOS modules.
+                This will be merged into the `specialArgs` used to evaluate
+                the NixOS configurations.
+              '';
+            };
+
             ephemeral = mkOption {
               type = types.bool;
               default = false;