experimental: Support LLB output format
Moby's BuildKit defines a low-level build language called LLB that
expresses image builds as a graph of source-code-to-binary artifacts. It
promises to make image building more efficient among many other things.
This is a first step in supporting LLB. It uses the docker2llb package
to convert the dockerfile compiler's output and is therefore
inefficient in its transcompilation. However, the actual resulting LLB
instructions should be reasonably efficient.