It seems silly in retrospect. I’d never have considered building an Arch Linux based Docker container in any other way but starting with one from the registry and a Dockerfile. But… it’s Arch Linux, you install this distro into a directory and chroot into it. Why settle for someone else’s old installation?
The script used to bootstrap an Arch install, pacstrap, even lets you exclude some packages from the default install or add things you need. So I ended up with a bash script that installs Arch into a directory, chroots into it and runs commands as required, then bundles the whole thing into a docker container. Repeatable, checked in to version control, and not wasting layers of AUFS.
Who needs a Dockerfile?