fix(docker): harden frontend runtime config

frontend/src/api/client.ts

@@ -1,4 +1,4 @@
-import { backendBaseUrl as BASE } from "./runtime-config";
+import { backendBaseUrl as BASE, joinBackendPath } from "./runtime-config";
 
 export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
   // Only declare a JSON content-type when we actually send a body. Body-less
@@ -10,7 +10,7 @@ export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T>
   if (init?.body != null && !("content-type" in headers) && !("Content-Type" in headers)) {
     headers["content-type"] = "application/json";
   }
-  const res = await fetch(`${BASE}${path}`, { ...init, headers });
+  const res = await fetch(joinBackendPath(BASE, path), { ...init, headers });
   if (!res.ok) throw new Error(`${res.status} ${await res.text().catch(() => "")}`);
   return res.status === 204 ? (undefined as T) : ((await res.json()) as T);
 }