Sign the published Docker images with cosign (#11129)

* ci: composite action that signs and verifies an image with cosign

Keyless, by digest, with a verification pass against the calling workflow's
own identity right after signing. Signatures use the .sig tag layout rather
than the OCI-referrer bundle cosign 3 writes by default, since that is what
the verifiers people run today read. Dependabot is pointed at the action so
the cosign-installer pin keeps moving.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker release: sign every variant on both registries

The merge job signs each variant's multi-arch index on GHCR and Docker Hub
once the tag exists, recursively so the platform images are covered too.
latest re-tags the same manifest and inherits the signature.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker dev: sign the dev image

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker latest: sign a latest rebuilt by hand

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker release: sign the foundationdb image

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker: sign the per-version foundationdb and rocksdb builds

They push to the same repository as the releases, so an admission policy
that verifies chrislusf/seaweedfs would otherwise reject them.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker: document image signature verification

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* ci: pin the actions the signing jobs newly run by commit

These run with registry credentials and the OIDC token that signs under
the repository's identity, so a retargeted tag upstream must not be able
to reach them.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker latest: pass the dispatch tag through env, not the script

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker: complete Kyverno policy, digest note, identity scope

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker latest: keep the dispatch tag out of the manifest script too

The step predates signing, but the job now holds the OIDC identity.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* ci: pin every action in the jobs that sign

The jobs that hold the OIDC identity run these with registry credentials,
so a retargeted tag upstream must not reach them.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker release: copy and sign the digest the run created, pin the rest

crane copy and the signature both resolved the tag, which another
publisher could move between the two steps. The index digest is read once,
right after it is created, and the Docker Hub copy and both signatures use
it. The manual latest rebuild gets the same treatment. The actions in these
jobs are pinned to commits, crane to v0.22.0 by checksum, and the sparse
checkout no longer keeps the token.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker latest: the signing job checks out the workflow's own commit

The job only assembles and signs manifests, so nothing there needs the
source_ref checkout; the local signing action now comes from the same
revision as the workflow file that calls it.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

* docker release: take the index digest from the create result

imagetools create writes the descriptor it pushed with --metadata-file
(buildx 0.32+, the runners ship 0.36), so the digest no longer comes from
re-resolving the tag even within the same step.

Claude-Session: https://claude.ai/code/session_01A5zMqzaUg1Snur4Yg8xJGa

.github/actions/sign-image/action.yml

@@ -0,0 +1,39 @@
+name: Sign container images
+description: >
+  Keyless cosign signature on each image, then a verification pass against the
+  identity the signature should carry, so a misconfigured job fails here and not
+  on someone's cluster. That identity is the calling workflow's own,
+  https://github.com/<owner>/<repo>/.github/workflows/<file>@<ref>.
+  The calling job needs `id-token: write` and a registry login for every image.
+
+inputs:
+  images:
+    description: Image references by digest (name@sha256:...), whitespace separated.
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Install cosign
+      uses: sigstore/cosign-installer@6f9f17788090df1f26f669e9d70d6ae9567deba6 # v4.1.2
+
+    - name: Sign
+      shell: bash
+      env:
+        IMAGES: ${{ inputs.images }}
+      run: |
+        set -euo pipefail
+        # The .sig tag layout: the OCI-referrer bundle cosign 3 writes by default
+        # is not read by the Kyverno and policy-controller releases in use today.
+        cosign sign --yes --recursive --new-bundle-format=false $IMAGES
+
+    - name: Verify
+      shell: bash
+      env:
+        IMAGES: ${{ inputs.images }}
+      run: |
+        set -euo pipefail
+        cosign verify \
+          --certificate-oidc-issuer https://token.actions.githubusercontent.com \
+          --certificate-identity "https://github.com/$GITHUB_WORKFLOW_REF" \
+          $IMAGES