Kubernetes DNS lookup issue and "invalid" in the /etc/resolv.conf file
As configured in CoreDNS ConfigMap default upstream nameservers are inherited from node, that is everything outside the cluster domain (.cluster.local)
So "invalid" is an entry copied from Node's /etc/resolv.conf file during Pod creation.
If you would manually modify /etc/resolv.conf on you Node, every Pod with dnsPolicy: ClusterFirst will inherit /etc/resolv.conf with this modification.
So, after adding --node-ip flag to kubelet and restarting CoreDNS Pods you should re-deploy your busybox Pod so it can inherit /etc/resolv.conf from the Node.