dockerTools: replace --no-clobber with --update=none

Since coreutils v9.2 the `--no-clobber` flag results in a non-zero exit
code when the destination files exist. Using `--update=none` will now
reproduce the old behavior of `--no-clobber`.

However, the `--update=none` flag was introduced in coreutils v9.3 and
thus `mergeImages` will fail if you have an older version than v9.3 in
stdenv after applying this commit.

[coreutils v9.3 changelog](https://github.com/coreutils/coreutils/blob/f386722dc0d996d5379f12b4a8d4dd15ca7df4b5/NEWS#L48)

pkgs/build-support/docker/default.nix

@@ -777,7 +777,7 @@ rec {
       fi
     done
     # Copy all layers from input images to output image directory
-    cp -R --no-clobber inputs/*/* image/
+    cp -R --update=none inputs/*/* image/
     # Merge repositories objects and manifests
     jq -s add "''${repos[@]}" > repositories
     jq -s add "''${manifests[@]}" > manifest.json