Rewrite cosign step following documentation (#168)

Signed-off-by: Federico Busetti <[email protected]>

Author: febus982

.github/workflows/ci-pipeline.yml

@@ -37,8 +37,6 @@ jobs:
       # https://github.com/sigstore/cosign-installer
       - name: Install cosign
         uses: sigstore/[email protected]
-        with:
-          cosign-release: 'v2.1.1'
 
       - name: Setup Docker buildx
         uses: docker/[email protected]
@@ -99,16 +97,23 @@ jobs:
           cache-from: type=gha
           cache-to: type=gha,mode=max
 
+      #TODO: Implement signature using generated key: https://docs.sigstore.dev/signing/quickstart/#signing-with-a-generated-key
+
       # 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
+      - name: Sign the published Docker image using GitHub OIDC Token
         env:
           # https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-an-intermediate-environment-variable
           TAGS: ${{ steps.meta.outputs.tags }}
           DIGEST: ${{ steps.build-and-push.outputs.digest }}
         # This step uses the identity token to provision an ephemeral certificate
         # against the sigstore community Fulcio instance.
-        run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}
+        run: |
+          images=""
+          for tag in ${TAGS}; do
+            images+="${tag}@${DIGEST} "
+          done
+          cosign sign --yes ${images}