Add new build rule for "staging" branch

.drone.yml

@@ -30,6 +30,25 @@ steps:
         exclude:
         - pull_request
 
+  - name: docker
+    image: plugins/docker
+    settings:
+      repo: docker.data.coop/data-coop-website
+      registry: docker.data.coop
+      username:
+        from_secret: DOCKER_USERNAME
+      password:
+        from_secret: DOCKER_PASSWORD
+      tags:
+        - "${DRONE_BUILD_NUMBER}"
+        - "dev"
+    when:
+      branch:
+        - staging
+      event:
+        exclude:
+        - pull_request
+
   - name: notify
     image: plugins/matrix
     settings: