Adds missing KUBERNETES_CLUSTER_DOMAIN environment variable to docs (#305)

* add cluster domain env var docs

* remove docker example

Author: maltesander

docs/modules/commons-operator/pages/reference/environment-variables.adoc

@@ -0,0 +1,21 @@
+= Environment variables
+
+This operator accepts the following environment variables:
+
+== KUBERNETES_CLUSTER_DOMAIN
+
+*Default value*: cluster.local
+
+*Required*: false
+
+*Multiple values*: false
+
+This instructs the operator, which value it should use for the Kubernetes `clusterDomain` setting.
+Make sure to keep this in sync with whatever setting your cluster uses.
+Please see the documentation xref:guides:kubernetes-cluster-domain.adoc[on configuring the Kubernetes cluster domain] for more information on this feature.
+
+[source]
+----
+export KUBERNETES_CLUSTER_DOMAIN=mycluster.local
+cargo run -- run
+----

docs/modules/commons-operator/pages/reference/index.adoc

@@ -3,7 +3,8 @@
 Consult the reference documentation section to find exhaustive information on:
 
 * Descriptions and default values of all properties in the CRDs used by this operator in the xref:reference/crds.adoc[].
+* The xref:reference/environment-variables.adoc[] accepted by the operator.
 
-== Command line parameters and environment variables
+== Command line parameters
 
-At the moment this operator accepts no command line parameters and does not read any environment variables.
+At the moment this operator accepts no command line parameters.