Fix required_contexts for 'docs' environment

Author: Noah Hanjun Lee

deploy.yml

@@ -1,8 +1,4 @@
 envs:
-  # local environment is used to test features.
-  - name: local
-    auto_merge: false
-
   - name: dev
     task: ${GITPLOY_DEPLOY_TASK:=rollback}:kubernetes
     # Disable "auto_merge" to avoid merge conflict
@@ -31,3 +27,4 @@ envs:
   - name: docs
     auto_merge: false
     description: Publish the documentation of Gitploy.
+    required_contexts: []