Is a Vagrant image similar to coreos  in plan?
I have managed to manually produce an "atomic-base-box". Starting from the VDI distributed by Project Atomic, create and boot a regular hand-made VM on VirtualBox (Fedora 64 type). Follow the steps below:
Create a "vagrant" user, password "vagrant":
Change root password to "vagrant"
Configure sudo with no password for "vagrant" user:
(insert the line below in the file visudo edits)
vagrant ALL=(ALL) NOPASSWD: ALL
Download the "vagrant.pub" file from https://github.com/mitchellh/vagrant/tree/master/keys and rename it to "authorized keys" and copy it into the VM as "~vagrant/.ssh/authorizedkeys" (crate folder ".ssh" if needed). You will have to adjust permissions after that (see below). DO IT AS "vagrant" (NOT root)! [[HINT: enable port forwarding in the VM on VirtualBox console, so you can scp/ssh into the VM, that will make copying files into it a breeze with a tool like WinSCP]].
[use scp whatever you have to copy the key file into the VM]
chmod 0700 ~vagrant/.ssh
chmod 0600 ~vagrant/.ssh/authorized_keys
Remove pesky demands of tty from "vagrant". Mind the single quotes so shell ignores the "!" thingie:
echo 'Defaults:vagrant !requiretty' > /etc/sudoers.d/999-vagrant-not-requiretty
chmod 440 /etc/sudoers.d/999-vagrant-not-requiretty
Uncomment/insert/edit these lines in /etc/ssh/sshd_config (as root or sudo). This is important or SSH login takes ages:
Shutdown the VM
Package the new base box:
vagrant package --base atomic-base --output atomic-base.box
(OPTIONAL) Remove a previously failed attempt that might be in the system:
vagrant box remove atomic-base virtualbox
Add the new base box to your system:
vagrant box add atomic-base atomic-base.box
Eventually all this could be done with "packer", but I am happy this way.