Keep original tag and only append sha256 digest

**Describe the problem/challenge you have**
[A description of the current challenge that you are experiencing.]
Sorry if I have missed this while checking the documentation, but is it possible to maintain the original tag and simply append the digest? The effect would be the same, while keeping the image's original version tag readable.

**Describe the solution you'd like**
[A clear and concise description of what you want to happen. If applicable a visual representation of the UX (ex: new CLI argument name, the behavior expected).]

For example:
`nginx:1.14.2`

Would become:

`index.docker.io/library/nginx:1.14.2@sha256:f7988fb6c02e0ce69257d9bd9cf37ae20a60f1df7563c3a2a6abe24160306b8d`

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start by reviewing kbld's current image-reference output and documentation for the nginx example. Trace where the image tag and sha256 digest are assembled, then verify that the original tag is preserved while the digest is appended; done means output matches the requested reference format and existing behavior remains intact.

Written by the indexing model from the issue text.