fix: reuse production images across worker services

docker/docker-compose.prod.yml

@@ -60,6 +60,9 @@ services:
 
   scheduler:
     <<: *php-service
+    # Reuse the server PHP image; building the same tag concurrently causes
+    # BuildKit export collisions during a full production bootstrap.
+    build: null
     command:
       - sh
       - -c
@@ -70,6 +73,8 @@ services:
 
   kb-worker:
     <<: *crawler-service
+    # Reuse the crawler image built by the crawler service.
+    build: null
     command:
       - python
       - scripts/kb_worker.py