Add Docker 1.12+ (service) based deployment (#9)

docker/dns.yml

@@ -0,0 +1,23 @@
+version: "3.2"
+
+services:
+  coredns:
+    image: coredns/coredns
+    command: -conf /data/Corefile
+    ports:
+      - "53:53/udp"
+      - "53:53/tcp"
+      - "9153:9153/tcp"
+    volumes:
+      - coredns:/data
+    deploy:
+      mode: global
+      placement:
+        constraints:
+          - "node.labels.iface != extern"
+      restart_policy:
+        condition: on-failure
+
+volumes:
+  coredns:
+    external: true