[CI] Better CI

The `.export.el' script will export automatically all my blog posts
from `content-org/blog.org' to multiple `.md' files in `content/'.
Manual export is no longer required!

.drone.yml

@@ -9,10 +9,11 @@ steps:
     - git submodule update --init --recursive
 
 - name: build
-  image: alpine
+  image: silex/emacs:27-alpine
   commands:
     - apk add hugo
     - mkdir blog
+    - emacs --script .export.el
     - hugo -d blog
 
 - name: deploy