fix(build): Containerfile builds again — relative module imports, all four pages and shared assets in the image

frontend/assets/app.js

@@ -63,7 +63,7 @@
  * All DOM ids match frontend/index.html.
  */
 
-import { fetchIsAdmin, initSharedHeader } from "/assets/header.js";
+import { fetchIsAdmin, initSharedHeader } from "./header.js";
 
 const messagesEl = document.querySelector("#messages");
 const emptyState = document.querySelector("#empty-state");