dockerTools: fix permissions on base image

If the base image has been built with nixpkgs.dockerTools, the image
configuration and manifest are readonly so we first need to change
their permissions before removing them.

Fix #27632.

pkgs/build-support/docker/default.nix

@@ -437,8 +437,8 @@ rec {
           echo "Unpacking base image..."
           tar -C image -xpf "$fromImage"
           # Do not import the base image configuration and manifest
+          chmod a+w image image/*.json
           rm -f image/*.json
-          rm -f image/manifest.json
 
           if [[ -z "$fromImageName" ]]; then
             fromImageName=$(jshon -k < image/repositories|head -n1)