Specifies whether to include the disk and what image to use.
Possible values are: - source-image: to use the same image that
was used to create the source instance's corresponding disk.
Applicable to the boot disk and additional read-write disks. -
source-image-family: to use the same image family that was used
to create the source instance's corresponding disk. Applicable
to the boot disk and additional read-write disks. - custom-
image: to use a user-provided image url for disk creation.
Applicable to the boot disk and additional read-write disks. -
attach-read-only: to attach a read-only disk. Applicable to
read-only disks. - do-not-include: to exclude a disk from the
template. Applicable to additional read-write disks, local SSDs,
and read-only disks.
