fix: CoreDNS | configure sequential nameserver usage

This is required for core dns to pick the correct nameservers as
configured by the host's resolv.conf file, according to
https://github.com/k3s-io/k3s/discussions/7822

core-dns/private-dns.yaml

@@ -0,0 +1,10 @@
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: coredns-custom
+  namespace: kube-system
+data:
+  forward.override: |-
+    forward . /etc/resolv.conf {
+      policy sequential
+    }