Create backup trusted container signing keys

[p5]

github.com/containers/image now supports multiple Sigstore container keys to be trusted for a single registry.

We should create a new keypair, ideally with the private key stored in a separate location to the existing one, and configure Podman to trust this new public key alongside the current one. This allows us to seamlessly rotate keys in the event of a leak/compromise.

Here's how I implemented it on my custom image
rsturla/eternal-main@8a579ee