Add Doc for `buildctl build --opts`

[arcosx]

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Fix issues:

• https://github.com/moby/buildkit/issues/1632
• https://github.com/moby/buildkit/issues/2625
• https://github.com/moby/buildkit/issues/1280

[arcosx]

I think we need a separate docs page that defines all the build options and context/local structure for the Dockerfile frontend. This can be used to build Dockerfiles with buildctl but also for people creating new frontends so that they can pick variable names that are consistent with Dockerfile frontend. We can also directly define how the docker buildx flags map to these options there.

Then this page can be linked from the main readme.

Yes, I think so, I Just move doc to docs/dockerfile-frontend-options.md, Any suggestion about this PR?

[daghack]

We've noticed it's been a while since there was movement on this PR, so we're going to close it for now. Feel free to reopen if efforts continue! :)