Summary

Adds SLSA build-provenance attestation to every non-snapshot image published by the shared docker workflows.

• New composite action actions/attest_image wraps the full attest+verify path: it lowercases the image ref once, calls actions/attest@v4.1.0 (pinned to 59d8942), and runs gh attestation verify against the just-pushed digest. Verify hard-fails on every consumer; the SAN policy accepts either the reusable-workflow signer (IABTechLab/uid2-shared-actions) or a composite-caller signer (see "Verify SAN policy" below).
• Both shared-publish-java-to-docker-versioned.yaml and actions/shared_publish_to_docker/action.yaml now call attest_image@v3 instead of inlining the attest block.
• Publish jobs gain id-token: write and attestations: write.
• Attestation is skipped on snapshot builds via the existing not_snapshot guard.

Closes UID2-6764. Spike was UID2-5763.

Smoke test evidence

1. Unit smoke (composite action in isolation)

Run 25542801315 — attest_image against a throwaway alpine image built inline. External verify:

$ gh attestation verify \
    "oci://ghcr.io/iabtechlab/uid2-shared-actions/test-attest@sha256:e008cbdd1c67eee898020ad96d56ff0d42d762585ef4c1153479abaf5a4112bb" \
    --owner "IABTechLab"
✓ Verification succeeded!

- Attestation #1
  - Build repo:..... IABTechLab/uid2-shared-actions
  - Build workflow:. .github/workflows/test-attest-image.yaml@refs/heads/bmz-UID2-6764-artifact-attestation
  - Signer repo:.... IABTechLab/uid2-shared-actions
  - Signer workflow: .github/workflows/test-attest-image.yaml@refs/heads/bmz-UID2-6764-artifact-attestation

2. End-to-end smoke (full shared workflow chain)

Run 25645229683 on UnifiedID2/uid2-test-source (private/internal), branch release-UID2-6764-smoke. Workflow conclusion: success, every step green including the in-CI gh attestation verify. Exercised in order: shared_publish_setup → docker login → docker metadata → docker build (with load: true) → actions/vulnerability_scan@v3 → docker push (push: true) → actions/attest_image (lowercase → actions/attest@v4.1.0 → in-CI verify) → actions/shared_create_releases@v2 (draft release).

Attestation created for ghcr.io/unifiedid2/uid2-test-source/uid2-6764-smoke@sha256:07b8a50e71f0b5d4731e37204709cdcc52a9d5a57c05508658356f1d771060d1, signed by GitHub's internal Sigstore instance, uploaded to both the GitHub attestations API and the OCI registry. No verify warning; the in-CI gh attestation verify hard-passes against the reusable-workflow signer.

Verify SAN policy

gh attestation verify enforces two things against a bundle: a source-repo-owner extension (set by --owner) and a signer-identity SAN regex. With only --owner, the SAN regex defaults to ^https://github.com/<owner>/, which silently rejects every reusable-workflow consumer — because the cert SAN for a reusable workflow is the reusable workflow's own URL, not the caller's repo (see cli/cli#9045 and gh attestation verify --help).

attest_image is invoked from two places:

1. The reusable workflow shared-publish-{,java-}to-docker-versioned.yaml — the reusable workflow itself is the signer, so SAN is under IABTechLab/uid2-shared-actions/.
2. The composite action actions/shared_publish_to_docker invoked directly by a caller — the caller's workflow is the signer, so SAN is under the caller's repo (${{ github.repository }}).

attest_image passes --cert-identity-regex "^https://github\.com/(IABTechLab/uid2-shared-actions|${{ github.repository }})/" to accept either pattern. There is no private-repo special case: the wrapper error Error: verifying with issuer "GitHub, Inc." returned by gh CLI swallows the underlying no matching certificate identity and reflects whichever Sigstore CA signed the bundle (public-good for public repos, GitHub's internal CA for private/internal repos) — neither is itself the blocker.

Test plan

• Snapshot smoke on IABTechLab/uid2-admin (Java path) — run 25421656856 — not_snapshot guard verified.
• Unit smoke of attest_image — run 25542801315 — sign + verify green.
• End-to-end smoke of the full non-Java shared workflow chain — run 25645229683 — green on a private/internal repo, in-CI verify hard-passes against the reusable-workflow signer.
• Release-tag E2E on each public consumer's first real publish after v3 is promoted; verified digests recorded in UID2-6764.

Post-merge sequence

1. Merge this PR.
2. Run update-major-version-tags.yaml on main immediately after — the refactored workflows reference actions/attest_image@v3, and consumers triggering between merge and tag promotion would fail with "action not found".
3. Merge each of the 6 caller-repo follow-up PRs below.
4. On each public consumer's first real publish, capture the verified digest in UID2-6764.
5. Delete IABTechLab/uid2-admin:bmz-UID2-6764-test (the snapshot smoke pin branch).

Caller-repo follow-up — already opened (one PR each, all open)

Each grants id-token: write + attestations: write (plus the implicit defaults the publish job already relied on). They're additive and harmless until this PR merges and v3 is promoted.

SDK images are explicitly out of scope; follow-up ticket to be filed separately.