feat: Add KUBERNETES_NODE_NAME env var to operator Pods (#537)

* feat: Add KUBERNETES_NODE_NAME env var to operator Pods

* docs: Mention clap arguments

* Update docs

Author: sbernauer

template/deploy/helm/[[operator]]/templates/deployment.yaml.j2

@@ -43,11 +43,26 @@ spec:
             - mountPath: /etc/stackable/{{ include "operator.appname" . }}/config-spec
               name: config-spec
           env:
+            # The following env vars are passed as clap (think CLI) arguments to the operator.
+            # They are picked up by clap using the structs defied in the operator.
+            # (which is turn pulls in https://github.com/stackabletech/operator-rs/blob/main/crates/stackable-operator/src/cli.rs)
+            # You can read there about the expected values and purposes.
+
+            # Sometimes products need to know the operator image, e.g. the opa-bundle-builder OPA
+            # sidecar uses the operator image.
             - name: OPERATOR_IMAGE
               # Tilt can use annotations as image paths, but not env variables
               valueFrom:
                 fieldRef:
                   fieldPath: metadata.annotations['internal.stackable.tech/image']
+
+            # Operators need to know the node name they are running on, to e.g. discover the
+            # Kubernetes domain name from the kubelet API.
+            - name: KUBERNETES_NODE_NAME
+              valueFrom:
+                fieldRef:
+                  fieldPath: spec.nodeName
+
             {{- if .Values.kubernetesClusterDomain }}
             - name: KUBERNETES_CLUSTER_DOMAIN
               value: {{ .Values.kubernetesClusterDomain | quote }}