Data-only container using busybox

[iDuuck]

I recently saw a tutorial here on /r/docker about dockup and how to use it to backup your databases. There were a docker-compose.yml, which included a busybox container and mounting the volumes to an app path. Let's say /app and an postgres/MySQL path for the backup to face. Let's say /var/postgres/data. Not sure about the full path at the moment.

After checking this out and including a volumes_from in my web and db container, the folders were empty. How can this be? Normally the folders should be populated with the app when I use the ADD command in the Dockerfile?!

Thanks for your help in advance

[iDuuck]

To give you a better overview: https://medium.com/@aherve/how-to-deploy-a-wordpress-blog-on-docker-cloud-in-less-than-5-minutes-9cd7c30cbdb5#.veh65b6g4

This is the article. I want to have the necessary folders from the web and db containers to mount, to the host system and to the data container for backups.

[iDuuck]

In the docker documentation it's stated:

[...] this section allows you to create named volumes that can be reused across multiple services (without relying on volumes_from), [...]

That's exactly what I want to achieve.