gke-disk-image-builder: Configurable attached disk source image

[nstogner]

Customer requirement: need the compute instance to run with a configurable disk source image.

Reference:

                                                        Disks: []*compute.AttachedDisk{
                                                                &compute.AttachedDisk{
                                                                        AutoDelete: true,
@@ -136,9 +169,10 @@ func GenerateDiskImage(ctx context.Context, req Request) error {
                                                                        DeviceName: fmt.Sprintf("%s-bootable-disk", name),
                                                                        Mode:       "READ_WRITE",
                                                                        InitializeParams: &compute.AttachedDiskInitializeParams{
-                                                                               DiskSizeGb:  req.DiskSizeGB,
-                                                                               DiskType:    fmt.Sprintf("projects/%s/zones/%s/diskTyp
es/%s", req.ProjectName, req.Zone, req.DiskType),
-                                                                               SourceImage: /* NEED THIS TO BE CONFIGURABLE */

[github-actions]

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