I found an interesting article that goes through the process of building a Ubuntu image with Docker. It basically builds a Dockerfile and outputs a .tar file which is then converted to an ISO (Source: https://slai.github.io/posts/customising-ubuntu-live-isos-with-docker/). I assume the ISO can then be converted using Packer or something.
Has anyone else managed to convert a Dockerfile to a MAAS-bootable TGZ?