This is a work base to introduce more features like build time
dockerfile optimisations, dependency analysis and parallel build, as
well as a first step to go from a dispatch-inline process to a
frontend+backend process.
Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>
rewritten from github.com/moby/moby 669c0677980b04bcbf871bb7c2d9f07caccfd42b