Have the starter docker-publish action sign digests.

This change installs `sigstore/cosign` using the `cosign-installer` action,
and uses sigstore's "keyless" signing process to sign the resulting image
digest using the action's identity token (see: `id-token: write`).

Signed-off-by: Matt Moore <mattomata@gmail.com>

ci/docker-publish.yml

@@ -29,11 +29,20 @@ jobs:
     permissions:
       contents: read
       packages: write
+      # This is used to complete the identity challenge
+      # with sigstore/fulcio when running outside of PRs.
+      id-token: write
 
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
 
+      # Install the cosign tool except on PR
+      # https://github.com/sigstore/cosign-installer
+      - name: Install cosign
+        if: github.event_name != 'pull_request'
+        uses: sigstore/cosign-installer@1e95c1de343b5b0c23352d6417ee3e48d5bcd422
+
       # Login against a Docker registry except on PR
       # https://github.com/docker/login-action
       - name: Log into registry ${{ env.REGISTRY }}
@@ -55,9 +64,21 @@ jobs:
       # Build and push Docker image with Buildx (don't push on PR)
       # https://github.com/docker/build-push-action
       - name: Build and push Docker image
+        id: build-and-push
         uses: docker/build-push-action@ad44023a93711e3deb337508980b4b5e9bcdc5dc
         with:
           context: .
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+
+      # Sign the resulting Docker image digest except on PR
+      # https://github.com/sigstore/cosign
+      - name: Sign the published Docker image
+        if: github.event_name != 'pull_request'
+        env:
+          COSIGN_EXPERIMENTAL: "true"
+        # This step uses the identity token to provision an ephemeral certificate
+        # against the sigstore community Fulcio instance, and records it to the
+        # sigstore community Rekor transparency log.
+        run: cosign sign --force ${{ steps.build-and-push.outputs.digest }}