Fix webpack asset patch with '/' as relative root

https://gitlab.com/gitlab-org/gitlab-ce/issues/60932

app/assets/javascripts/lib/utils/webpack.js

+import { joinPaths } from '~/lib/utils/url_utility';
+
 // tell webpack to load assets from origin so that web workers don't break
 // eslint-disable-next-line import/prefer-default-export
 export function resetServiceWorkersPublicPath() {
@@ -5,7 +7,7 @@ export function resetServiceWorkersPublicPath() {
   // the webpack publicPath setting at runtime.
   // see: https://webpack.js.org/guides/public-path/
   const relativeRootPath = (gon && gon.relative_url_root) || '';
-  const webpackAssetPath = `${relativeRootPath}/assets/webpack/`;
+  const webpackAssetPath = joinPaths(relativeRootPath, '/assets/webpack/');
   __webpack_public_path__ = webpackAssetPath; // eslint-disable-line camelcase
 
   // monaco-editor-webpack-plugin currently (incorrectly) references the

changelogs/unreleased/fix-webpack-assets-relative-url-bug.yml

+---
+title: Fix webpack assets handling when relative url root is '/'
+merge_request: 27909
+author:
+type: fixed