nixos/containers: warn if containers are used but disabled

This is a follow-up to 8dfe8e447e.

nixos/modules/virtualisation/nixos-containers.nix

@@ -771,8 +771,13 @@ in
   };
 
 
-  config = mkIf (config.boot.enableContainers) (let
+  config = mkMerge [
+    {
+      warnings = optional (!config.boot.enableContainers && config.containers != {})
+        "containers.<name> is used, but boot.enableContainers is false. To use containers.<name>, set boot.enableContainers to true.";
+    }
 
+    (mkIf (config.boot.enableContainers) (let
       unit = {
         description = "Container '%i'";
 
@@ -905,7 +910,8 @@ in
         "tap"
         "tun"
       ];
-  });
+    }))
+  ];
 
   meta.buildDocsInSandbox = false;
 }