Only build docker image on master.

.drone.yml

@@ -1,17 +1,26 @@
 ---
 kind: pipeline
-name: default
 type: docker
+name: default
+
+platform:
+  os: linux
+  arch: amd64
 
 steps:
 - name: docker
   image: plugins/docker
   settings:
     auto_tag: true
+    password:
+      from_secret: docker_password
     registry: registry.huefox.com
     repo: registry.huefox.com/foxbot
     username:
       from_secret: docker_username
-    password:
-      from_secret: docker_password
+
+trigger:
+  branch:
+  - master
+
 ...