Architecture decisions
Registry cache entries are single-layer OCI image manifests
The registry cache backend stores each cache entry (task outputs) as a standard OCI image manifest with a single tar layer, not as an OCI artifact with a custom media type. A cache entry therefore looks like an ordinary one-layer image, so it works on every registry (Docker Hub, GHCR, ECR, GAR, Artifactory, plain registry:2) and reuses the image push/pull plumbing already built for package <registry>.
Considered options
- OCI artifact (custom
mediaType) — rejected: cleaner semantics, but older/stricter registries reject unknown media types, costing the portability that is the entire reason to use a registry as a cache. No concrete need for artifact typing.
Consequences
- Mild lock-in: changing the format later invalidates existing cached entries — acceptable, since caches are regenerable.
- Revisit only if a concrete need for OCI-artifact typing appears.