actions: update cosign from github starter workflow

Author: timhawes

.github/workflows/docker-publish.yml

@@ -90,7 +90,9 @@ jobs:
       - name: Sign the published Docker image
         if: ${{ github.event_name != 'pull_request' }}
         env:
-          COSIGN_EXPERIMENTAL: "true"
+          # https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-an-intermediate-environment-variable
+          TAGS: ${{ steps.meta.outputs.tags }}
+          DIGEST: ${{ steps.build-and-push.outputs.digest }}
         # This step uses the identity token to provision an ephemeral certificate
         # against the sigstore community Fulcio instance.
-        run: echo "${{ steps.meta.outputs.tags }}" | xargs -I {} cosign sign {}@${{ steps.build-and-push.outputs.digest }}
+        run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}