If your images use kvm+qcow2 you can just spin up new vms as a delta image using qcow2's support for a backing store.

I want a new instance of WEBSERVER.qcow2?

    qemu-img create -b WEBSERVER.qcow2 -t qcow2 WEBSERVER-$SERIALNUMBER.qcow2 

If you're doing this as a PaaS or for CI, you do this as part of your new image creation and then pass in the new qcow2 to your vm (maybe via libvirt). If you aren't doing this or something very similar, you're spinning your wheels and wasting time/resources.

Other benefits: docker images are basically tarballs, which means they are much smaller.

And, importantly, Docker maintains a filesystem-level diff between versions of an image, and only needs to transmit each diff once. So you get tremendous bandwidth savings when transmitting multiple images created from the same base.