Build Docker image and push to Docker Hub on master

.drone.yml

@@ -8,4 +8,13 @@ steps:
   image: golang
   commands:
   - go get
-  - go build
+  - go build
+- name: docker
+  image: plugins/docker
+  settings:
+    repo: pocketjawa/discord-pocketbot
+    auto_tag: true
+    username:
+      from_secret: docker_username
+    password:
+      from_secret: docker_password