fix: run cities labeler from its release image

Pull git.cosocial.info/cosocial-canada/atproto-cosocial-labeler instead of
cloning and building on the server, which took several minutes per deploy.
Watchtower picks up new :latest releases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

templates/cities-labeler-compose.yaml.j2

@@ -5,7 +5,7 @@ name: cities-labeler
 
 services:
   app:
-    build: .
+    image: {{ pds_cities_image | default('git.cosocial.info/cosocial-canada/atproto-cosocial-labeler:latest') }}
     env_file: .env
     ports:
       - "127.0.0.1:4100:4100"