feat: publish signed image provenance manifests

[msarahan]

Posted by Codex (GPT-5.6) on behalf of Michael Sarahan. Treat this description as LLM-generated.

Summary

Add a signed, OCI-referrer RAPIDS image provenance manifest for published base
and notebooks image digests.

• Export exact conda-meta package facts through minimal BuildKit targets rather
  than downloading image layers.
• Attach a platform-specific manifest to every pushed architecture image, then
  attach a multiarch index manifest that links those platform records.
• Record the image digest, platform, source SHA, workflow URL, build arguments,
  and conda package name/version/build/channel/source URL.
• Keylessly sign each attached OCI artifact with GitHub Actions OIDC and Cosign.
• Keep pURLs explicitly unmapped until a verified upstream mapping is available;
  this intentionally does not invent pkg:conda/... identifiers.

Relationship to #805

#805 is complementary. It generates and
embeds a CycloneDX SBOM in the final image. This change makes digest-bound build
facts discoverable as registry metadata and supplies the platform-to-multiarch
relationship needed by scan triage without pulling image layers.

If #805 lands, the preferred follow-up is to add its SBOM digest/reference to
this manifest rather than maintain a separate provenance model. Its Dockerfile
stage renaming will require a small conflict resolution in this branch's
provenance-base and provenance-notebooks export targets.

Validation

• pytest -q tests/test_image_provenance_manifest.py
• pre-commit run --all-files (Ruff, ShellCheck, Zizmor, copyright checks)
• YAML parsing and git diff --check

Buildx static validation was attempted but the local configured remote builder
timed out while connecting. No image was built locally. The first CI run should
therefore confirm Docker Hub OCI-referrer support and keyless Cosign signing on
the production runners.

[msarahan]

/ok to test a9fec54

[msarahan]

superseded by #896 because CI can't run from my fork