Using ONBUILD COPY --from with BuildKit enabled causes build process to fail

[mikecsmith]

Problem Statement

Docker has now enabled BuildKit by default in builds >= 2.4.0.0 (Released 2020-09-30).

BuildKit has a known issue (moby/buildkit#816) which breaks all builds using ONBUILD COPY --from=<imagename> directives because it runs builds concurrently and attempts to COPY --from before compiled artefacts exist.

This breaks the runtime image which contains an ONBUILD COPY --from=0 directive to extract the compiled executable from a build container.

Suggested Solution

Update the README to add a Known Issues section and reference the fact that these images will only work if BuildKit is disabled. Update the example build command to prefix it with DOCKER_BUILDKIT=0 docker build ...

Addititonal Information

Please monitor moby/buildkit#816 to track potential permanent fixes to this issue.