dind: split DNS -- ts.net via MagicDNS, everything else via Cloudflare

The host resolves the tailnet through MagicDNS on tailscale0 and everything
else through Cloudflare (systemd-resolved split DNS). MagicDNS has no
upstream for names outside the tailnet and answers them SERVFAIL, so
handing 100.100.100.100 to job containers as their first resolver made
every public lookup wait on a failure before falling back: image pulls
stalled and the e2e lane's *.localtest.me fetches blew a 5s poll.

A CoreDNS forwarder at a fixed address on the runner network now answers
both: ts.net is forwarded to MagicDNS, the rest to 1.1.1.1/1.0.0.1.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018j9YG8YFjcsVLnqzNz7LmA

Corefile

@@ -0,0 +1,19 @@
+# Split-horizon resolver for the runner's Docker daemon and every job
+# container it starts.
+#
+# The host resolves the tailnet through MagicDNS on tailscale0 and everything
+# else through Cloudflare on the default link (systemd-resolved split DNS).
+# MagicDNS itself has no upstream configured for names outside the tailnet and
+# answers them with SERVFAIL, so handing 100.100.100.100 to job containers as
+# their first resolver made every public lookup (image pulls, *.localtest.me
+# in the e2e lanes) wait on a failure before falling back -- long enough to
+# time out a 5s Playwright poll. Only ts.net goes to MagicDNS here.
+ts.net:53 {
+    errors
+    forward . 100.100.100.100
+}
+.:53 {
+    errors
+    cache 30
+    forward . 1.1.1.1 1.0.0.1
+}