Add configuring cluster domain guide (#673)

* initial cluster domain docs

* add note in kubernetes section for cluster domain

* fix link

* fixes

* Apply suggestions from code review

Co-authored-by: Sebastian Bernauer <[email protected]>

* rename to kubernetes-cluster-domain.adoc

* fix typo

* review comments

* remove auto-dectection explanation

* Update modules/guides/pages/kubernetes-cluster-domain.adoc

Co-authored-by: Sebastian Bernauer <[email protected]>

---------

Co-authored-by: Sebastian Bernauer <[email protected]>
Co-authored-by: Sebastian Bernauer <[email protected]>

Author: maltesander

modules/ROOT/pages/kubernetes/index.adoc

@@ -77,3 +77,8 @@ Kubernetes control plane is running at https://127.0.0.1:6443
 CoreDNS is running at https://127.0.0.1:6443/api/v1/namespaces/kube-system/services/kube-dns:dns/proxy
 Metrics-server is running at https://127.0.0.1:6443/api/v1/namespaces/kube-system/services/https:metrics-server:https/proxy
 ----
+
+=== Configuring the cluster domain
+
+In case a non-default cluster domain is used as described in https://kubernetes.io/docs/tasks/administer-cluster/dns-custom-nameservers/[Customizing DNS Service],
+Stackable operators can be configured accordingly. This is described in detail in the xref:guides:kubernetes-cluster-domain.adoc[Configuring the Kubernetes cluster domain] guide.

modules/guides/nav.adoc

@@ -4,3 +4,4 @@
 ** xref:running-stackable-in-an-airgapped-environment.adoc[]
 ** xref:viewing-and-verifying-sboms.adoc[]
 ** xref:enabling-verification-of-image-signatures.adoc[]
+** xref:kubernetes-cluster-domain.adoc[]

modules/guides/pages/kubernetes-cluster-domain.adoc

@@ -0,0 +1,15 @@
+= Configuring the Kubernetes cluster domain
+:description: Configure Stackable operators to use a different cluster domain other than 'cluster.local'.
+:dns-custom-nameservers: https://kubernetes.io/docs/tasks/administer-cluster/dns-custom-nameservers/
+:dns-pod-service: https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/
+
+Stackable operators allow the configuration of a non-default cluster domain as described in {dns-custom-nameservers}[Customizing DNS Service] (and more in {dns-pod-service}[DNS for Services and Pods]).
+
+The cluster domain can be configured using an environment variable `KUBERNETES_CLUSTER_DOMAIN` set on the operators.
+This environment variable can be configured via the helm values property `kubernetesClusterDomain` during the installation of the operators.
+
+```
+helm install <product>-operator stackable-stable/<product>-operator --set kubernetesClusterDomain="my-cluster.local"
+```
+
+If the environment variable `KUBERNETES_CLUSTER_DOMAIN` (or the helm property `kubernetesClusterDomain`) are not set / overriden, the operator will default the cluster domain to `cluster.local`.