Re-add and apply MDL rule MD040

We were following it about half of the time and we even added it to our
former `.mdlrc` file. However, for some reason, MDL doesn't detect this
rule when specified in the `.mdlrc` file, so we didn't notice we weren't
following it in many cases.

Now that we're using a style file to configure MDL, we can enable this
rule again and apply it, since now MDL correctly includes it in its
report.

docs/en/installation/docker.md

@@ -91,7 +91,7 @@ POSTGRES_PASSWORD=password docker-compose up -d database
 
 You can now initialize your development DB and populate it with:
 
-```
+```bash
 POSTGRES_PASSWORD=password docker-compose run app rake db:create db:migrate
 POSTGRES_PASSWORD=password docker-compose run app rake db:dev_seed
 ```