nixos/nixos-containers: actually eliminate costs if no containers are used (#427001)

doc/release-notes/rl-2511.section.md

@@ -154,6 +154,9 @@
   and called `setup.py` from the source tree, which is deprecated.
   The modern alternative is to configure `pyproject = true` with `build-system = [ setuptools ]`.
 
+- `boot.enableContainers` is only turned on when a declarative NixOS container is defined in `containers`.
+  If you use the `nixos-container` tool for imperative container management, set `boot.enableContainers = true;` explicitly.
+
 ### Deprecations {#sec-nixpkgs-release-25.11-lib-deprecations}
 
 - Create the first release note entry in this section!

nixos/modules/virtualisation/nixos-containers.nix

@@ -518,10 +518,10 @@ in
 
     boot.enableContainers = mkOption {
       type = types.bool;
-      default = true;
+      default = config.containers != { };
+      defaultText = lib.literalExpression "config.containers != { }";
       description = ''
-        Whether to enable support for NixOS containers. Defaults to true
-        (at no cost if containers are not actually used).
+        Whether to enable support for NixOS containers.
       '';
     };