ci: sign the published image in CI via OIDC-federated KMS

[robinnsc]

What

A sign-image workflow: dispatch it with a verified image digest and an
approved run signs that digest on Docker Hub with the production KMS key —
attached Cosign signature, legacy tag-based storage, no transparency log
(deliberate, see below). AWS access is OIDC-federated (id-token: write);
there are no new secrets anywhere. cosign is pinned by checksum, the
credentials action by commit SHA. A post-sign gate fails the run unless the
sha256-<digest>.sig TAG exists and is a single OCI manifest — the exact
failure mode cosign v3's defaults produce (new-format referrer, no tag),
which we hit live on ECR tonight.

Why

Manual signing requires one person to simultaneously hold a production KMS
session and the team registry credential on a laptop. Nobody should be
comfortable with that. This moves the act into the dockerhub environment,
which supplies the registry credential it already holds and enforces the
two-person rule mechanically (required reviewers, no self-review, main
only) instead of by chat message.

The AWS side is already live and verified: ExtendDBContainerSigningCI
can be assumed only via GitHub OIDC by runs of this repository in the
dockerhub environment, and the key policy (both its Allow and its
explicit Deny) admits exactly that role plus the manual break-glass
operator role. This is Phase 1 of the pipeline consolidation plan, pulled
forward — nothing here is launch-throwaway.

Transparency log is off this release for consistency with the ECR
signature (signed from a corp machine, which cannot reach Rekor — it is
DNS-sinkholed). Runners can reach Rekor, so next release this becomes a
one-flag change plus runbook update.

Testing done

• yaml.safe_load passes; input gate follows the env-only pattern from
  ghcr-mirror (digest regex is the same one fuzzed on ci: mirror Cosign signature artifacts through the GHCR mirror job #264).
• The exact cosign flag set (--use-signing-config=false --new-bundle-format=false --tlog-upload=false) was proven live against
  ECR Public tonight: produced the .sig tag, single OCI manifest,
  simplesigning layer, verified with the production key.
• The IAM trust chain was verified by read-back (role trust condition,
  key policy Allow + Deny); policy before/after archived in release
  evidence.
• Not executed in Actions pre-merge (main-only environment, as with ci: add minimal GHCR candidate mirror job #256,
  ci: mirror Cosign signature artifacts through the GHCR mirror job #264, ci: accept promoted release tags in the GHCR mirror job #266). First dispatch is the live verification; every gate fails
  closed.

Checklist

• All tests pass (cargo test --workspace) — not applicable, workflow-only change
• Code is formatted (cargo fmt --check) — not applicable
• Clippy is clean (cargo clippy -- -W clippy::pedantic) — not applicable
• I have added or updated tests for new functionality — gates traced above; post-sign tag gate is the regression test for the storage-mode trap
• I have updated documentation if behavior changed — header documents rationale, trust chain, and the Rekor plan; runbook update follows merge
• Breaking changes are noted below (if any)

ADR / RFC: n/a — CI tooling only.

Breaking changes

None. The manual signing path remains available as break-glass.

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