Add option to sign image

Skopeo or Cosign can be used to sing a contaienr image, which should be added:

• https://github.com/containers/skopeo/blob/main/docs/skopeo-copy.1.md#options
• https://github.com/containers/skopeo/blob/main/docs/skopeo-generate-sigstore-key.1.md
• https://trivy.dev/latest/docs/supply-chain/attestation/sbom/
• https://github.com/sigstore/cosign