Nginx fails to start when upstream services aren’t available at startup time, showing host not found in upstream. This post focuses on fixing this issue in Docker first, then extends the solution to Kubernetes and Azure Container Apps. The error occurs because Nginx resolves all hostnames at startup, not request time. This fail-fast behaviour is reasonable in many environments, but problematic in container orchestration where services start in unpredictable order or may not always be available by design.