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

frontend/assets/login.js

@@ -17,7 +17,7 @@
  * All DOM ids match frontend/login.html.
  */
 
-import { fetchIsAdmin, initSharedHeader } from "/assets/header.js";
+import { fetchIsAdmin, initSharedHeader } from "./header.js";
 
 const form = document.querySelector("#login-form");
 const passwordInput = document.querySelector("#login-password");