This is a simple maintenance container, which responds to request at any URL with a maintenance announcement. It is available on G5's (non-public) Google Container Registry at gcr.io/g5-images/maintenance-page.
This is really intended to run in Kubernetes, using the provided sample Deployment in deployment.yaml.
The intention here is that a K8s service pointed at an app can be re-pointed with selectors to a pod running the maintenance page when the app is undergoing maintenance. That's it.
K8s exhibits some funky behavior when you attempt to kubectl apply a modified service definition (it merges the labels instead of replaces them). You can use kubectl replace, but that only works with the absolutely latest revision of the service definition. This behavior seems undefined, though, and could have dire consequences for a load balancer if the service is torn down while this happens.
As far as I can tell, you can't just check in two service definitions, one for normal behavior and one for maintenance mode, and reliably apply one or the other. The safest/sketchiest way to bring up a maintenance page is to either kubectl get svc/myservicename -O yaml" and edit it, or just kubectl edit`. I say sketchy, because if you misspell a selector, your site is now down.