Oh you have two installs of libvirt here. One in the Ubuntu image that's running the job. This one lacks qemu-img. The second install is the Docker container you've built which contains Fedora and has qemu-img. I can't tell which libvirt install your error is coming from, but I'd guess the "host" Ubuntu one which lacks qemu-imgignorant me, you're right!!! Ran also into issues for host/container libvirt pool discrepancies earlier on. Highly likely it's the host lacking qemu-img.
Confirming, cured it. Heaps of thanks for review!
-- All best! Matthias