docker-bake.hcl: emit attestation manifests as OCI artifacts (set oci-artifact=true) #123

Images published by this repo carry SBOM and SLSA provenance attestations (attest = ["type=provenance,mode=max", "type=sbom"] in docker-bake.hcl). However, the resulting attestation manifests are emitted in BuildKit's legacy Docker format rather than as OCI 1.1 artifacts.

As a result, the attestations are not discoverable through the OCI Referrers API, as they are missing both artifactType and the subject backlink to the parent image manifest.

Same issue and fix as in cloudnative-pg/cloudnative-pg#10601 and cloudnative-pg/postgres-extensions-containers#198.