Digest for locally built image

My operations team would like to prevent a newly built image from being pushed to repository if it duplicates what is already there. I was planning to have the CI/CD pipeline compare the local image digest with the digest of the image in the repository and push only on change.

Unfortunately, the locally built images don’t appear to have a digest. Is it possible to add this digest? Is the digest generation documented, so that I can determine what the digest would be?