Hi, and thanks a lot for this useful action 🙂 .

I was experimenting a bit with it and I have a question. From an end-user perspective, i.e., someone who is about to pull an image and use it locally, what's the recommended flow to verify it? I see there's an independent workflow for verifying images (docker/github-builder/.github/workflows/verify.yml@v1), but I'd like some help to understand what is it verifying, and how one can simulate it locally.

For example, I've built ghcr.io/apyrgio/dangerzone-test-github-builder@sha256:8eb497e92ed7071fc925e306214a6368bb2a3c90939ce15123d09471d3ba5a66 using this workflow.

Looking at the build job of this workflow, in the "Set result output" step, I see the following build output:

{
  "verifyCommands": "cosign verify --experimental-oci11 --new-bundle-format --certificate-oidc-issuer https://token.actions.githubusercontent.com/ --certificate-identity-regexp ^https://github.com/docker/github-builder/.github/workflows/build.yml.*$ ghcr.io/apyrgio/dangerzone-test-github-builder@sha256:c40cc89132c63ef82310cf1a2c05a6ad656ec88b9a9d29246825b470f1354a57",
  "imageDigest": "sha256:8eb497e92ed7071fc925e306214a6368bb2a3c90939ce15123d09471d3ba5a66",
  "artifactName": "",
  "signed": true
}

It seems that the verifyCommands field includes the commands that a user can run locally to verify the container image. What I'm not sure about is what we are verifying here.

In this case, the cosign verify command verifies the sha256:c40cc89132c63ef82310cf1a2c05a6ad656ec88b9a9d29246825b470f1354a57 manifest, which includes an in-toto attestation for the sha256:37ca0e50004ab4b4a51e6f87e6d9528a7d000e0a3702ff0a3c59181f7a558443 manifest, which is basically the image data for the linux/amd64 platform.

But the user will not pull this digest. They will instead pull either the root one, the platform specific one, or the image tag. For all the above, if they to run cosign verify ... against them, they will get the following:

Error: no signatures found
error during command execution: no signatures found

Should there be signatures for these digests?

Finally, the provided cosign verify ... command ensures that the image was created by this GitHub action. What's much more interesting though is the other attestation attributes, such as the repository or the source commit hash. If a user wants to verify against those, should they use the cosign verify ... command, or switch to cosign verify-attestation ... and pass a CUE policy or similar?