A built (as in `docker build && docker run`) image does not have a digest because the digests are for the tar+gzipped versions of the layers (+ the image config

A built (as in `docker build && docker run`) image does not have a digest because the digests are for the tar+gzipped versions of the layers (+ the image config and manifest). The layers are not tar+gzipped until you attempt to push them, at which point the digest is calculated.