feat: Add attestation entry point to Docker Images

[fabergat]

Description

This PR adds the support to the attestation verification in offline mode and give a suggestion on how to override the Docker entry point for Docker and Docker Compose.

Closes: #1310

Note:

The key material in trusted_root.jsonl does not have a built-in expiration date, so anything signed before you generate the trusted root file will continue to successfully verify. Anything signed after the file is generated will verify until that Sigstore instance rotates its key material, which typically happens a few times per year. You will not know if key material has been revoked since you last generated the trusted root file.

Open questions:

• Is this thing adding a lot more security?
• Is it worth it complicate the config?

Changes

Testing Information

Checklist:

• I have performed a self-review of my code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules