improve coredns health check, enable health plugin

root/defaults/Corefile

@@ -1,4 +1,5 @@
 . {
     loop
+    health
     forward . /etc/resolv.conf
 }

root/etc/s6-overlay/s6-rc.d/svc-coredns/run

@@ -3,9 +3,14 @@
 if netstat -apn | grep -q ":53 "; then
   echo "Another service is using port 53, disabling CoreDNS"
   sleep infinity
-else
+elif grep -q "health" /config/coredns/Corefile; then
+  exec \
+    s6-notifyoncheck -d -n 300 -w 1000 -c "redirfd -w 1 /dev/null curl -s http://localhost:8080/health" \
+      cd /config/coredns \
+      /app/coredns -dns.port=53
+else  
   exec \
     s6-notifyoncheck -d -n 300 -w 1000 -c "nc -z -u 127.0.0.1 53" \
-        cd /config/coredns \
-        /app/coredns -dns.port=53
+      cd /config/coredns \
+      /app/coredns -dns.port=53
 fi