Use mdformat container (#3)

Reviewed-on: #3

.drone.yml

@@ -3,9 +3,8 @@ kind: pipeline
 
 steps:
   - name: linting
-    image: python
+    image: docker.atomaka.com/mdformat
     commands:
-      - pip install -r requirements.txt
       - mdformat --check _posts/
   - name: jekyll-build
     image: ruby