download the root disk image itself with this. There were a bunch of missing bits to make this nice though:
First of all, for raw disk images we need to attach them to a loopback block device, to make them mountable. Easy-peasy, systemd-dissect --attach already delivers that.
Then, for tar disk images we need to bind mount the downloaded and unpacked image to /sysroot/ (which is where the rootfs goes before we transition into it).