'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

@@ -74,6 +74,9 @@ steps:
       from_secret: dockerhub_username
     password:
       from_secret: dockerhub_password
+    custom_dns:
+      - 128.142.17.5
+      - 128.142.16.5
 
 - name: publish-docker-revad-latest
   pull: always
@@ -86,6 +89,9 @@ steps:
       from_secret: dockerhub_username
     password:
       from_secret: dockerhub_password
+    custom_dns:
+      - 128.142.17.5
+      - 128.142.16.5
 
 - name: publish-docker-revad-eos-latest
   pull: always
@@ -98,6 +104,9 @@ steps:
       from_secret: dockerhub_username
     password:
       from_secret: dockerhub_password
+    custom_dns:
+      - 128.142.17.5
+      - 128.142.16.5
 
 ---
 kind: pipeline
@@ -223,6 +232,9 @@ steps:
       from_secret: dockerhub_username
     password:
       from_secret: dockerhub_password
+    custom_dns:
+    - 128.142.17.5
+    - 128.142.16.5
 
 - name: docker-revad-tag
   pull: always
@@ -235,6 +247,9 @@ steps:
       from_secret: dockerhub_username
     password:
       from_secret: dockerhub_password
+    custom_dns:
+    - 128.142.17.5
+    - 128.142.16.5
 
 - name: docker-revad-eos-tag
   pull: always
@@ -247,6 +262,9 @@ steps:
       from_secret: dockerhub_username
     password:
       from_secret: dockerhub_password
+    custom_dns:
+    - 128.142.17.5
+    - 128.142.16.5
 
 ---
 kind: pipeline