The outage that teaches you deployment A service passes every test locally. It fails in staging because the API calls localhost:5432 for Postgres — but Postgres is in another container, reachable only as db:5432 . This is not a Docker problem. It is a boundary problem: your code assumed an environment it does not own. Engineers who have shipped on AWS Lambda already avoid a class of these mistakes. They never SSH into a function to hot-fix. They inject config at deploy time. They ...