title: How is digest of Docker image calculated?
body: <p>A digest is the sha256 hash of a docker image, but an image is not really a single file but rather a set of layers.  I assumed the digest was the sha256 hash over the Image manifest file, but I have computed the sha256 hash of numerous manifest files and compared the result to the digest docker provide for the image and they are diff.  So what exactly is being sha256-hashed to create the Image digest value?</p>