ImageSource is used to describe build source that will be extracted from an image. A reference of
type ImageStreamTag, ImageStreamImage or DockerImage may be used. A pull secret can be specified
to pull the image from an external registry or override the default service account secret if pulling
from the internal registry. A list of paths to copy from the image and their respective destination
within the build directory must be specified in the paths array.

EXPERIMENTAL.  This will be changing to an array of images in the near future and no migration/compatibility 
will be provided.  Use at your own risk.