Build Attestation
A signed statement about how an artifact was produced — by which builder from which source — that a deployment gate can verify rather than trust.
A signature proves an artifact has not been altered since signing. An attestation proves more: it is a signed claim about the artifact's provenance — this digest was built by this builder, from this commit, with these dependencies, at this time.
That extra content is what makes a policy expressible. A cluster can be configured to admit only images with a valid attestation from the organisation's builder, referencing a commit on a protected branch. An image built on someone's laptop and pushed to the registry then fails admission, without anybody having to notice it.
The chain has to be complete to be worth anything. An attestation generated by a build that pulled dependencies from floating tags attests to a process, not to contents; one signed with a key the build itself can read attests to whoever compromised the build. Provenance is only as strong as the weakest of build isolation, key custody and the verification step actually being enforced rather than merely produced.