Doesn't work with DOCKER_BUILDKIT=1 and heredocs

[budney]

I'm using INCLUDE+ to include a Dockerfile that includes heredocs, which means that I'm setting DOCKER_BUILDKIT=1 before invoking docker build. I'm getting the same error as when I run the included Dockerfile without setting DOCKER_BUILDKIT: namely, docker build is choking on the contents of the heredoc because it's trying to parse them as if they're part of the Dockerfile.

My best guess as to what's going on is that heredocs are processed at some early stage of Dockerfile parsing, and the INCLUDE+ directive is getting parsed later -- so when it interpolates the contents of the included file, they're not being parsed correctly. Not being expert in ruby OR Docker, that's about as far as I can get in tracking down the problem.