The Debian project provides virtual machine disk images suitable for use
with Forrest.
They can be downloaded from cdimage.debian.org/images/cloud.
The correct images to use are the *-generic-amd64.raw variants,
e.g. debian-12-generic-amd64.raw.
$ curl --location --compressed -o debian-12-generic-amd64.raw \
https://cdimage.debian.org/images/cloud/bookworm/latest/debian-12-generic-amd64.raw