Add docker image upload to CI

.drone.yml

@@ -9,3 +9,22 @@ steps:
     volumes:
       - name: gopath
         path: /go
+
+  - name: publish
+    image: plugins/docker
+    settings:
+      auto_tag: true
+      target: final
+      registry:
+        from_secret: docker_registry
+      repo:
+        from_secret: docker_repo
+      username:
+        from_secret: docker_username
+      password:
+        from_secret: docker_password
+    when:
+      event:
+        - push
+        - tag
+      branch: master