Allow flattening etc images by digest

[imjasonh]

flatten and estargz fetch an external public image, modify it and serve it. They currently don't support fetching the external image by digest, because the way to express that would be in the image digest, and that digest in these cases would refer to the flattened/estargzed image, not the original.

Support flatten.kontain.me/ubuntu:sha256-abcdef... as a convention for referring to ubuntu@sha256:abcdef..., which can then be flattened, etc.