Every week or so, a new version of the Official Ubuntu Trusty x64 Vagrant base box for Virtual appears on Hashicorp Atlas: https://atlas.hashicorp.com/ubuntu/boxes/trusty64
Presumably, for repeatable consistency, this has all been automated.
I know there are general blog posts about how a Vagrant base box can be built but I'm more interested in how these boxes are built. I'm hoping, that like the rest of the Ubuntu ecosystem I've encountered, the process for building these boxes is all public open source.
I'm specifically interested in the options chosen during the Ubuntu OS installation and any post-OS-install changes that are made before packaging the box.
The details of using Vagrant, Packer, or other tools to package the machine into a box are much less important.