[mdelillo]

> I also want the CI builds to create my base boxes with packer in multiple steps.

This is certainly possible using concourse, I've done it myself on a few teams. We had one job that started with a base ISO and used the virtualbox-iso builder to apply updates and build an OVA. Then a second job would trigger whenever a new OVA was built and used the virtualbox-ovf to apply a different set of provisioners. Since the virtualbox-ovf builder uses an OVA/OVF file as both it's input and output, you can do that as many times as you'd like.