add linting rules for undeclared args in from

[daghack]

This adds another linting rule related to unmatched ARGs in the FROM command.

[tonistiigi]

Because this behavior is normally caught in the parser,

You mean the cases where build already errors? I don't think we need special handling for these cases or to skip any actual errors. I think it would be enough if we can detect:

ARG tag=foo
FROM busybox:${tag}${version} AS b

This is not invalid build but the $version part is something that is unhandled always and should cause a warning.

[daghack]

Exactly what I meant, yes.
Sounds good! I will update the tests to get rid of the logic skip and test that, instead. :)

[tonistiigi]

Looks like some errors in CI for latest version.

[daghack]

Okay, review ready again at last! 😂