You can extend the .NET runtime to add additional functionality by customizing the Docker image. For more information, see [Custom Runtimes](https://docs.cloud.google.com/appengine/docs/flexible/custom-runtimes). ## Health checking By default, all programs running in the flexible environment receive [health check requests](https://docs.cloud.google.com/appengine/docs/flexible/how-instances-are-managed#health_checking), and you don't need to write any additional code.