Dockerfile,Makefile: use a compressed tarball

When building the image for publishing in the infrastructure, we need a
rootfs that's <100MB, due to github filesize restrictions (this, plus
the fact that there's no git-lfs support for building in the docker
library toolchain). Compress the rootfs image so that it can be
hosted on github.

Dockerfile

 FROM scratch
-ADD archlinux.tar /
+ADD archlinux.tar.xz /
 ENV LANG=en_US.UTF-8
 CMD ["/usr/bin/bash"]

Makefile

@@ -9,7 +9,7 @@ rootfs:
 	arch-chroot $(TMPDIR) locale-gen
 	arch-chroot $(TMPDIR) pacman-key --init
 	arch-chroot $(TMPDIR) pacman-key --populate archlinux
-	tar --numeric-owner --xattrs --acls --exclude-from=exclude -C $(TMPDIR) -c . -f archlinux.tar
+	tar --numeric-owner --xattrs --acls --exclude-from=exclude -C $(TMPDIR) -c . -xf archlinux.tar.xz
 	rm -rf $(TMPDIR)
 
 docker-image: rootfs