The users run their own operating system, but the image must be
It contains a default disk attached, as the boot image will be copied to this disk.
The default disk is mounted on / so if you want to attach any additional disks, you have to choose a different mounting point.
A /image.raw file is used as "boot disk". This /image.raw is copied to the first attached volume of the vm. Cloud-init will take care of resizing the filesystem on the image to take the full disk size allocated in the deployment.