Hi Gary,
I would advise you use Packer. It is a well done open-source project that is made to generate images for a lot of platforms. It has lots of builders to generate images (Docker, lxc/lxd, AMIs, etc) and multiple provisioners to configure those. You can for instance run the LXC builder to create an LXC container and run the shell provisioner to configure your image. Or change the provisioner to Ansible to do the same.
In your case, behind the scenes, it will indeed start an lxc container and run a shell script (in the case of the shell provisioner) or Ansible. This makes it easy to switch from one type of runtime to another.