Add git_dir, web_dir as a named volume

[socketwench]

Right now Dropwhale does not preserve the state of the key directories as a volumes. This means that both the locally cloned repository git_dir (/tmp/drupal) and the web_dir (/var/www/html) are lost when the containers are stopped.

Add both of these as named volumes in docker-compose.yml, and/or using the VOLUME statement in .dropwhale/web/Dockerfile.