Add `--attest` and/or `--sbom`

[Bryce Lampe]

Some users would like to scan every stage of their build pipelines for vulnerabilities. Current solutions are limited to either

• inspecting only the final/pushed image, which usually only includes built artifacts and not dependencies; or
• stitching together a non-trivial chain of builds -- e.g. output stages to a local directory; run a command to scan those stages; reload from disk and push if the command succeeds.

BuildKit natively supports generating software-bill-of-materials on the fly, and we can expose that functionality via an attest (or sbom -- shorthand for --attest=type=sbom) option on Image.

A hypothetical policy-as-code could then ensure --sbom is applied to all images via a remediation policy. All images would then include a SBOM manifest appropriate for scanning.

Additional thoughts:

• Users may want to specify a different generator image, so we should at least support attest.
• Unless a target includes BUILDKIT_SBOM_SCAN_STAGE it will not be scanned by default, which is easy to forget and therefore error-prone. The provider currently parses the Dockerfile and knows all of the stages it's going to build, and it would be feasible to automatically inject BUILDKIT_SBOM_SCAN_STAGE into all/some targets if it's not already present. How would we enable or configure that behavior?