Extract dockerfile2llb

[vdemeester]

The dockerfile2llb frontend shouldn't live in this repository. It should live somewhere in the docker organization.

This issue is here to keep track of the work on that.

[AkihiroSuda]

Do we keep Dockerfile tests ( #183 ) in this repo after eliminating dockerfile2llb from this repo?

[tonistiigi]

@AkihiroSuda I think tests in this repo should refer to a fixed version of dockerfile frontend and any update in this repo would check it out and also run dockerfile test suite. If some other frontends get popular we could do the same for them to make sure there are no accidental regressions.

Atm, I think we can keep adding tests to this repo and move them out together.

[vdemeester]

@AkihiroSuda I would think Dockerfile tests would move to the dockerfile2llb repository

[tonistiigi]

We could start by moving github.com/moby/moby/builder/dockerfile/instructions github.com/moby/moby/builder/dockerfile/parser . For example, I would have liked to make a quick change for https://github.com/moby/buildkit/pull/196/files#diff-6f2add87df719e471c72e28c36caff8fR590 but moby implementation itself doesn't necessarily need it.