- Fixes#736.
- BuildKit returns nil ref for Dockerfiles containing only metadata
(`ENV/ARG/LABEL`) directives without filesystem operations
(`RUN/COPY/ADD`). Previously, this caused builds to fail with "no build
directives" error.
- Builder-shim 0.6.3 [fixes this](https://github.com/apple/container-builder-shim/pull/47) by
creating a minimal marker layer when ref is nil but image config is
valid, satisfying OCI manifest requirements.
- Also, added some tests for this behavior.