Add support for a Docker builder

Add support for a Docker builder

Adding a Docker builder will allow us to more easily test build images
locally in a container before pushing a patch to production to build
the VMs themselves. While not guarenteed to be exactly the same it's
a close enough approximation that many issues can be caught before
patches are pushed.

Additionally this allows projects to also use the same packer templates
to build container based builder nodes if there's any projects using
Docker for their Jenkins jobs.

Issue: RELENG-1052
Change-Id: If89801cea16046900e7e880265ad859a96595fa1
Signed-off-by: Thanh Ha <zxiiro@gmail.com>