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

docker-compose.yml

@@ -1,4 +1,17 @@
 services:
+  # Split DNS for the dind daemon and its job containers: ts.net -> MagicDNS,
+  # everything else -> Cloudflare. See Corefile for why MagicDNS alone fails.
+  dns:
+    image: coredns/coredns@sha256:af8c8d35a5d184b386c4a6d1a012c8b218d40d1376474c7d071bb6c07201f47d
+    container_name: gitea-runner-dns
+    restart: unless-stopped
+    command: ["-conf", "/Corefile"]
+    volumes:
+      - ./Corefile:/Corefile:ro
+    networks:
+      runner:
+        ipv4_address: 172.28.0.53
+
   # Isolated Docker daemon for the runner's job containers.
   # Keeps the host Docker clean and gives reliable cache networking.
   dind:
@@ -37,12 +50,14 @@ services:
     # own network, and a killed job leaves one behind. 10.210/16 in /24s is 256
     # networks and collides with nothing here (no 10.x route on the host; the
     # tailnet is 100.64/10).
-    command: ["dockerd", "--host=tcp://0.0.0.0:2375", "--host=unix:///var/run/docker.sock", "--host=unix:///dind-sock/docker.sock", "--tls=false", "--dns=100.100.100.100", "--dns=1.1.1.1", "--default-address-pool", "base=10.210.0.0/16,size=24"]
+    command: ["dockerd", "--host=tcp://0.0.0.0:2375", "--host=unix:///var/run/docker.sock", "--host=unix:///dind-sock/docker.sock", "--tls=false", "--dns=172.28.0.53", "--dns=1.1.1.1", "--default-address-pool", "base=10.210.0.0/16,size=24"]
     environment:
       DOCKER_TLS_CERTDIR: ""
     dns:
+      - 172.28.0.53
       - 1.1.1.1
-      - 1.0.0.1
+    depends_on:
+      - dns
     networks:
       - runner
     volumes:
@@ -92,6 +107,9 @@ services:
 
 networks:
   runner:
+    ipam:
+      config:
+        - subnet: 172.28.0.0/24
 
 volumes:
   dind-storage: