Enable keyless signing for private repos. (#1295)

Now that cosign 1.4 is out, we can perform keyless signing without panicking on private images (and without `--force` uploading to Rekor).

Signed-off-by: Matt Moore <mattmoor@chainguard.dev>

ci/docker-publish.yml

@@ -42,6 +42,9 @@ jobs:
       - name: Install cosign
         if: github.event_name != 'pull_request'
         uses: sigstore/cosign-installer@1e95c1de343b5b0c23352d6417ee3e48d5bcd422
+        with:
+          cosign-release: 'v1.4.0'
+
 
       # Workaround: https://github.com/docker/build-push-action/issues/461
       - name: Setup Docker buildx
@@ -76,18 +79,15 @@ jobs:
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
 
-      # Sign the resulting Docker image digest except on PRs and private repos
-      # The keyless signing process records signatures on the Rekor public
-      # transparency log, so signing is disabled for private repos by default
-      # to avoid leaking private data.  If you wish to sign things anyways,
-      # then this check can be removed and --force can be added to the cosign
-      # command below.
+      # Sign the resulting Docker image digest except on PRs.
+      # This will only write to the public Rekor transparency log when the Docker
+      # repository is public to avoid leaking data.  If you would like to publish
+      # transparency data even for private images, pass --force to cosign below.
       # https://github.com/sigstore/cosign
       - name: Sign the published Docker image
-        if: ${{ github.event_name != 'pull_request' && !github.event.repository.private }}
+        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.
+        # against the sigstore community Fulcio instance.
         run: cosign sign ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build-and-push.outputs.digest }}