Adds missing KUBERNETES_CLUSTER_DOMAIN environment variable to docs. #1791

Job	Run time
Check if committed README is the one we would render from the available parts	28s
Run Cargo Deny (advisories)	44s
Check if committed Helm charts are up to date	38s
Run Cargo Deny (bans licenses sources)	38s
Run Clippy	32s
Run RustDoc	1m 2s
Run Rustfmt	14s
Run Cargo Tests	34s
Run Cargo Udeps	1m 20s
Select target helm repository based on action trigger	1s
All tests passed	1s
Package Charts, Build Docker Image and publish them - ubuntu-latest	11m 5s
Package Charts, Build Docker Image and publish them - ubicloud-standard-8-arm	9m 8s
Build and publish manifest list	55s
Run the OpenShift Preflight check on the published images	19s
	27m 39s

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