Mention dependency of app service over db service

Author: kmehant

containers/python-3-postgres/.devcontainer/docker-compose.yml

@@ -14,10 +14,13 @@ services:
     # Overrides default command so things don't shut down after the process ends.
     command: sleep infinity
 
+    links: 
+      - db
+    
   db:
     image: postgres
     restart: always
     ports: 
       - 5432:5432
     environment:
-      POSTGRES_PASSWORD: LocalPassword
+      POSTGRES_PASSWORD: LocalPassword