CI: split up dockerfile frontend tests more

[tonistiigi]

Currently longest-running items in CI are:

• build / test (./frontend/dockerfile, dockerfile) (pull_request) Successful in 23m
• build / test (./frontend/dockerfile) (pull_request) Successful in 21m
• build / cross (pull_request) Successful in 11m
• build / test-os (windows-latest) (pull_request) Successful in 8m
• build / image (pull_request) Successful in 8m
• ...

So the top 2 items take >2x the rest. As these are tests they also block other steps. It might be good to try to split them more into pieces that run in parallel(ideally 4).

@crazy-max

[crazy-max]

Agree, will work on that. I also wonder if we should start thinking about moving the dockerfile-frontend to a dedicated repo, maybe keep tests to keep track of regressions.

[crazy-max]

Oh looks like it was already in the pipe #163

[tonistiigi]

Oh looks like it was already in the pipe #163

I conceptually agree but still worried that this will cause maintenance issues. For testing this does not matter though, even if a bunch of these tests would be in a separate repo we would still need to invoke them on BuildKit PR as they provide a lot to the test coverage.