WIP attempt to verify docker image tree

bash verify-image-digests <image-name> [digest]

See get_docker_root_digest function to know where to take root digest

Though digest argument is optional if you want just to verify tree

If you beleive root digest is uncopromised then no need to specify it in args

TODO:

• Manifest canonicalization?
• tar vs tar.gz layers
• Relate local index.json/manifest.json with distribution manifest