initial implementation

docs/administrator.md

@@ -429,6 +429,10 @@ from numerous escape characters in the latter log entry, view it in CLI with
 `PodTemplate` used by the operator is yet to be updated with the default values
 used internally in K8s.
 
+The operator also support lazy updates of the Spilo image. That means the pod template of a 
+PG cluster's stateful set is updated immediately with the new image, but no rolling upgrade follows. This feature saves you 
+some downtime when you know pods are re-started after the update anyway, for instance due to the node rotation.
+
 ## Logical backups
 
 The operator can manage K8s cron jobs to run logical backups of Postgres

docs/reference/operator_parameters.md

@@ -75,6 +75,10 @@ Those are top-level keys, containing both leaf keys and groups.
   [OpenAPI v3 schema validation](https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/#validation)
   The default is `true`.
 
+* **enable_lazy_image_upgrade**
+  Instruct operator to update only the statefulsets with the new image without immediately doing the rolling update. The assumption is pods will be re-started later with the new image, for example due to the node rotation.
+  The default is `false`.
+
 * **etcd_host**
   Etcd connection string for Patroni defined as `host:port`. Not required when
   Patroni native Kubernetes support is used. The default is empty (use