The previous approach wrote the host.docker.internal IP to the k3s
resolv.conf, assuming it would serve DNS. On Docker Desktop, this IP
is reachable but doesn't run a DNS server, causing CoreDNS to fail
resolving external names and breaking the PKI job and Helm install.
The new approach discovers Docker's embedded DNS listener ports from
the DOCKER_OUTPUT iptables chain, then sets up DNAT rules to forward
DNS traffic from k3s pods (via PREROUTING) to Docker's actual DNS
resolver on 127.0.0.11. The container's eth0 IP is used as the
nameserver in the k3s resolv.conf since it's routable from all pods.