ci: sign the released images with cosign (#963)

Published images carry no signature, so there is no way for a user to verify
that the image they pull was built by this workflow from this repository.

Sign them keyless, using the Github OIDC token, so verification needs no key
material on either side. The manifest is signed by digest rather than by tag:
every tag produced for a release points at the same digest, and the same
digest is valid in both registries the build pushes to, so signing each tag
would only repeat the same signature.

Verify with:

    cosign verify ghcr.io/donkie/spoolman:<version> \
      --certificate-identity-regexp '^https://github\.com/Donkie/Spoolman/\.github/workflows/ci\.yml@refs/' \
      --certificate-oidc-issuer https://token.actions.githubusercontent.com

Assisted-By: Claude <noreply@anthropic.com>

Signed-off-by: Aleksei Sviridkin <f@lex.la>

.github/workflows/ci.yml

@@ -393,6 +393,8 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       packages: write
+      # Lets cosign fetch an OIDC token from Github and sign without a private key.
+      id-token: write
     steps:
       - name: Checkout
         uses: actions/checkout@v6
@@ -452,7 +454,15 @@ jobs:
           echo "GIT_COMMIT=$(git rev-parse --short HEAD)" >> "$GITHUB_ENV"
           echo "BUILD_DATE=$(date -u +"%Y-%m-%dT%H:%M:%SZ")" >> "$GITHUB_ENV"
 
+      - name: Install cosign
+        uses: sigstore/cosign-installer@v4.1.2
+        with:
+          # Installer v4+ verifies cosign v3 downloads via the Sigstore bundle
+          # format; pin an explicit, current cosign release for reproducibility.
+          cosign-release: v3.1.1
+
       - name: Build and push Docker images
+        id: build
         uses: docker/build-push-action@v6.7.0
         with:
           context: .
@@ -469,6 +479,24 @@ jobs:
             type=registry,ref=ghcr.io/${{ env.GHCR_IMAGE }}:buildcache-amd64
             type=registry,ref=ghcr.io/${{ env.GHCR_IMAGE }}:buildcache-arm64
             type=registry,ref=ghcr.io/${{ env.GHCR_IMAGE }}:buildcache-armv7
+
+      # Sign the manifest by digest rather than by tag: all the tags above point
+      # at the same digest, and the same digest is valid in both registries, so
+      # signing every tag would just repeat the same signature.
+      - name: Sign the published images
+        env:
+          DIGEST: ${{ steps.build.outputs.digest }}
+          TAGS: |
+            ${{ steps.meta-ghcr.outputs.tags }}
+            ${{ steps.meta-dh.outputs.tags }}
+        run: |
+          echo "${TAGS}" | while read -r tag; do
+            [ -n "${tag}" ] || continue
+            echo "${tag%:*}"
+          done | sort --unique | while read -r repository; do
+            echo "Signing ${repository}@${DIGEST}"
+            cosign sign --yes "${repository}@${DIGEST}"
+          done
   #
   # Create Github Release if CI was triggered by a tag
   #