This initContainer will then allow us to bootstrap the folder structure needed to add our required mutability, while leveraging vanilla CloudNativePG.

It's a webhook in the K8s API server that will call a specific URI and route configured in a MutatingWebhookConfiguration config which will inject an initContainer into a Pod.

There are two types of Admission Controllers: Validating and Mutating. This is a Mutating controller.

Mutating admission controllers take in Kubernetes resource specifications and return an updated resource specification. They modify the resource attributes before they are passed into subsequent phases. They also perform side-effect calculations or make external calls (in the case of custom admission controllers).