Sign the images with sigstore's fulcio/rekor
The ecosystem is moving towards sigstore and we are federated with the public fulcio instance[1], so let's sign our images. Cosign is not used, but the sigstore feature built into podman, which works basically the same way as cosign.
[1] https://github.com/sigstore/fulcio/pull/1214
Fix #77 (closed)