feat(release): sign and attest published docker images (#480)

Every published Sencho Docker image is now signed with Sigstore cosign
using GitHub Actions keyless OIDC, and ships with an embedded SBOM plus
SLSA provenance attestation attached as OCI referrers on the image.
Users can verify the signature and inspect the attestations themselves
before pulling an image into production.

  cosign verify saelix/sencho:<tag> \
    --certificate-identity-regexp "https://github.com/AnsoCode/Sencho/.*" \
    --certificate-oidc-issuer https://token.actions.githubusercontent.com

The release pipeline now also publishes a new moving minor tag
(saelix/sencho:X.Y, e.g. 0.42) alongside the existing latest and
immutable X.Y.Z tags, so operators who want the latest patch on a given
minor line can pin without chasing every release. Since every 0.x minor
is potentially breaking, the {{major}}-only tag is deferred until 1.0.

cosign is installed via the sigstore/cosign-installer action pinned to
a full commit SHA. The signing step batches every tag produced by
metadata-action into a single cosign invocation pinned to the built
digest, so all tags resolve to the same manifest and we save N-1
Rekor/Fulcio round trips. The job declares id-token: write so the
ambient OIDC token can be minted.

A new docs page (docs/reference/verifying-images.mdx) walks users
through installing cosign, verifying an image, and inspecting the SBOM
and provenance with docker buildx imagetools. A note calls out that
signatures start shipping with v0.43.0, so older images failing
verification is expected behavior and not tampering.

.github/workflows/docker-publish.yml

@@ -17,6 +17,8 @@ jobs:
     timeout-minutes: 30
     permissions:
       contents: read
+      # Required for cosign keyless signing via GitHub OIDC.
+      id-token: write
     steps:
       - name: Check out the repo
         uses: actions/checkout@v6
@@ -35,16 +37,27 @@ jobs:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
+      - name: Install cosign
+        uses: sigstore/cosign-installer@cad07c2e89fa2edd6e2d7bab4c1aa38e53f76003 # v4.1.1
+
       - name: Extract metadata (tags, labels) for Docker
         id: meta
         uses: docker/metadata-action@v6
         with:
           images: saelix/sencho
+          # On a v-tag push we publish:
+          #   latest           always points at the newest release
+          #   X.Y.Z            the immutable semver tag
+          #   X.Y              moving minor tag for users who want latest patch
+          # The pre-1.0 constraint hides the {{major}}-only tag until we ship 1.0,
+          # since on 0.x every minor is potentially breaking.
           tags: |
             type=raw,value=latest,enable=${{ startsWith(github.ref, 'refs/tags/v') }}
             type=semver,pattern={{version}},enable=${{ startsWith(github.ref, 'refs/tags/v') }}
+            type=semver,pattern={{major}}.{{minor}},enable=${{ startsWith(github.ref, 'refs/tags/v') }}
 
       - name: Build and push Docker image
+        id: build
         uses: docker/build-push-action@v7
         with:
           context: .
@@ -54,3 +67,32 @@ jobs:
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=registry,ref=saelix/sencho:buildcache
           cache-to: type=registry,ref=saelix/sencho:buildcache,mode=max
+          # SBOM + provenance attestations are embedded as OCI referrers on the
+          # published image. Inspect with: docker buildx imagetools inspect <img>
+          sbom: true
+          provenance: mode=max
+
+      - name: Sign published image with cosign (keyless)
+        # Signs every tag produced by metadata-action using the ambient GitHub
+        # OIDC token. No private keys, no secrets. Users verify with:
+        #   cosign verify saelix/sencho:<tag> \
+        #     --certificate-identity-regexp "https://github.com/AnsoCode/Sencho/.*" \
+        #     --certificate-oidc-issuer https://token.actions.githubusercontent.com
+        env:
+          TAGS: ${{ steps.meta.outputs.tags }}
+          DIGEST: ${{ steps.build.outputs.digest }}
+        run: |
+          # Build a single cosign invocation with every tag pinned to the digest.
+          # All tags resolve to the same manifest, so one call signs them all and
+          # saves N-1 Rekor/Fulcio round trips. On workflow_dispatch $TAGS is empty
+          # and the refs array stays empty, so we no-op cleanly.
+          refs=()
+          while IFS= read -r tag; do
+            [ -n "$tag" ] || continue
+            refs+=("${tag}@${DIGEST}")
+          done <<< "$TAGS"
+          if [ ${#refs[@]} -gt 0 ]; then
+            cosign sign --yes "${refs[@]}"
+          else
+            echo "No tags to sign (likely a workflow_dispatch run on a non-tag ref)."
+          fi