Installer: Make Private network optional

Change-Id: Ic7a2e5250a42dc03de2416b1e2a0d1bbca3f010c
diff --git a/core/installer/values-tmpl/env-dns.cue b/core/installer/values-tmpl/env-dns.cue
index 99941be..13cc217 100644
--- a/core/installer/values-tmpl/env-dns.cue
+++ b/core/installer/values-tmpl/env-dns.cue
@@ -154,7 +154,7 @@
 			}
 			config: "coredns.conf"
 			db: "records.db"
-			zone: global.domain
+			zone: networks.public.domain
 			publicIP: strings.Join(global.publicIP, ",")
 			privateIP: global.network.ingress
 			nameserverIP: strings.Join(global.nameserverIP, ",")