docs: add CLUSTER_DOMAIN documentation to env-variables.md (#2409)

Since it's only documented in the helm values file and not in this file.

Author: ruant

docs/env-variables.md

@@ -11,4 +11,5 @@ Operator behavior can be customized using environment variables in the `minio-op
 |OPERATOR_STS_ENABLED| This toggles the STS Service on or off                                                                                                                                                                 | `on`, `off`                 | `on`                            |
 |OPERATOR_STS_AUTO_TLS_ENABLED| Env variable name to turn on and off generating the STS TLS certificate automatically using CSR. If it is disabled, you must provide a certificate issued externally                                                    | `on`, `off`                 | `on`                            |
 |WATCHED_NAMESPACE| The namespaces which the operator watches for MinIO tenants. Defaults to `""` for all namespaces.                                                                                                      |                         |                                 |
-|MINIO_OPERATOR_IMAGE| This variable controls the image of the minio instance's sidecar and validate-arguments. If not set, the mirrors of the minio instance's sidecar and validate-arguments use the operator's image. | "" | "" |
+|MINIO_OPERATOR_IMAGE| This variable controls the image of the minio instance's sidecar and validate-arguments. If not set, the mirrors of the minio instance's sidecar and validate-arguments use the operator's image. | "" | "" |
+|CLUSTER_DOMAIN| Controls the cluster name to use when "building" the full DNS name that the operator uses to access the tenant instances (for example for health checks). | "my-cluster.company.com" | "cluster.local" |