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You mean something like posting signed hashes in a public place? So you can verify what you got is what you want?

Do you mean formal verification? Verifying such a vast web service may be nearly impossible, especially since you'd still have to rely on a database server, os, and kernel, the most popular of which are seemingly fundamentally incompatible with verification.

This is what the Notary portion of Harbor does:

https://github.com/goharbor/harbor/blob/master/docs/use_nota...

https://docs.docker.com/notary/getting_started/

You can only have 100% verifiable container images if the Dockerfiles uploaded by users are reproducible (in the sense of https://reproducible-builds.org/). The vast majority probably aren't, and I'm not sure the Hub could reliably detect those that are.