`short_id` and `id` for local image (`DockerClient.images.get()`) are not the image SHA

Hi,

I've noticed that documentation is slightly off when it comes to id/short_id for the images. id/short_id for registry images are the same as the digests for that images. For local ones however, it can differ. For local images, id/short_id returns ID of the JSON file returned in docker inspect command. It would be good to have an option to get digest of the local image which points out to proper image from the registry.