I had thought this took a while, but I left the VM running one night and ended up with a full size qcow2 image.
Filling up empty space with zeroes (could be run from a container image): $ docker run -it --rm -v /:/host alpine chroot /host /bin/sh -c 'dd if=/dev/zero of=/var/lost+found/zeroes; rm /var/lost+found/zeroes'
Freeing the space that was zeroed (with docker stopped): $ qemu-img convert -O qcow2 Docker.qcow2 Docker-slim.qcow2 $ mv Docker-slim.qcow2 Docker.qcow2