dockerfile: add buildkit syntax reference

[crazy-max]

depends on moby/buildkit#2902 so keeping in draft

Fetch dockerfile reference from buildkit repo as well as the buildkit syntax in https://github.com/crazy-max/buildkit/tree/dockerfile-ref/frontend/dockerfile/docs (use my fork for now):

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

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maxtrem271991 left a comment:

example

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[dockertopia]

@crazy-max I'm fine with publishing this asap to have it public somewhere as a starting point, but I think we're adding a lot of information into reference guides (e.g. this whole overview) that should be in the Manuals. References should be more like cheat sheets than dictionaries or Wikipedia articles. I also think that we need a product-pov (in how we organize the topics as well as) in naming features, options etc. instead of repo/project view.
I have also noticed that we call examples to items which are just samples and sometimes we don't even say what the sample illustrates.

[crazy-max]

@crazy-max I'm fine with publishing this asap to have it public somewhere as a starting point, but I think we're adding a lot of information into reference guides (e.g. this whole overview) that should be in the Manuals. References should be more like cheat sheets than dictionaries or Wikipedia articles. I also think that we need a product-pov (in how we organize the topics as well as) in naming features, options etc. instead of repo/project view. I have also noticed that we call examples to items which are just samples and sometimes we don't even say what the sample illustrates.

@dockertopia This PR is still draft. The idea here is to break reference in subsections but still need more work as you said. Can you look at #14948 instead? This is the one that just fetch the dockerfile reference docs from buildkit repo so we are aligned.

[crazy-max]

clsoing this one since #14948 has been merged. will open a new one for new reference location when dockerfile changelog will be there.