Handle signing OCI artifacts in *ARTIFACT_OUTPUTS

[bradbeck]

Changes

Handle signing OCI artifacts that are hinted using *ARTIFACT_OUTPUTS

Fixes #1575

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Release Notes

Sign OCI artifacts hinted by `*ARTIFACT_OUTPUTS` and `isbuildArtifact: true`

[arewm]

I previously opened an issue that Chains should stop signing type-hinted artifacts: #1346

If that is implemented, would we just have to back this out as well?

[anithapriyanatarajan]

I previously opened an issue that Chains should stop signing type-hinted artifacts: #1346

If that is implemented, would we just have to back this out as well?

@arewm - If the user intends to use Chains for image signing as well, this PR is needed. If the user intends not to sign the image artifact through the Chains controller, this will not take effect based on the configmap value as done with #1419 This PR handles the issue that you have created. Hope this helps

[jkhelil]

@bradbeck please add release note

[jkhelil]

what happens if artifact is declared via both IMAGE_URL/IMAGE_DIGEST and ARTIFACT_OUTPUTS ?
are falling in double signing ?

[bradbeck]

What is the current behavior when both IMAGE_URL/IMAGE_DIGEST & IMAGES define the same artifact?

[anithapriyanatarajan]

@bradbeck Thank you for this PR. I have verified the double signing behavior for both combinations:
IMAGE_DIGEST + IMAGE_URL + IMAGES results in duplicate .sig layers (2 identical payloads, 2 independent signatures)
IMAGE_DIGEST + IMAGE_URL + ARTIFACT_OUTPUTS results in duplicate .att layers (same attestation stored twice)
Filed as [#1596] with reproduction steps and oras/cosign output confirming both cases.

The root cause on initial analysis with Claude is no deduplication in [ExtractOCIImagesFromResults] which is pre-existing and this PR has surfaced it by adding a third loop. I'd suggest merging this PR as-is and tracking the deduplication fix as a separate follow-up.

@jkhelil , would you prefer a follow-up PR for the deduplication? The test cases would need to cover each type-hint combination independently and in combination, so it is cleaner to handle separately rather than expanding the scope of this PR. Thank you.

[ab-ghosh]

The duplicate signing concern from overlapping result formats is addressed in #1601

[ab-ghosh]

/lgtm

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