'custom_dns' in docker build steps to workaround dind DNS issues

Ref. drone-plugins/drone-docker#193
Note: this disables cloud.drone.io from building the images, as
the alternative DNS servers are internal-only

.drone.yml

@@ -30,6 +30,9 @@ steps:
       from_secret: dockerhub_username
     password:
       from_secret: dockerhub_password
+    custom_dns:
+      - 128.142.17.5
+      - 128.142.16.5
 
 ---
 kind: pipeline
@@ -59,3 +62,6 @@ steps:
       from_secret: dockerhub_password
     build_args:
       - VERSION=${DRONE_TAG}
+    custom_dns:
+      - 128.142.17.5
+      - 128.142.16.5