Refactor validate for GitHub Actions

[crazy-max]

This PR is the beginning of a best-effort to migrate to GitHub Actions since travis-ci.org will be officially closed down completely on December 31st, 2020. Migrate to travis-ci.com will require a limited number of credits:

...we will upgrade you to our trial (free) plan with a 10K credit allotment (which allows around 1000 minutes in a Linux environment).

While waiting to switch to GitHub Actions, we should consider contacting Travis about their OSS credits plan:

We will be offering an allotment of OSS minutes that will be reviewed and allocated on a case by case basis. Should you want to apply for these credits please open a request with Travis CI support stating that you’d like to be considered for the OSS allotment. Please include:

• Your account name and VCS provider (like travis-ci.com/github/[your account name] )
• How many credits (build minutes) you’d like to request (should your run out of credits again you can repeat the process to request more or discuss a renewable amount)

Ref.

• Switch to GitHub Actions moby/buildkit#1736
• Refactor hack scripts moby/buildkit#1821
• Refactor validate for GitHub Actions moby/buildkit#1824
• Refactor generators moby/buildkit#1829
• Refactor test for GitHub Actions moby/buildkit#1831
• Refactor cross for GitHub Actions moby/buildkit#1833
• Refactor release for GitHub Actions moby/buildkit#1835

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

[tonistiigi]

Is this intentionally master?

[crazy-max]

@tonistiigi Same behavior as Travis: https://travis-ci.org/github/docker/buildx/jobs/747558356#L176 but we can change it if you think so.

[tonistiigi]

I think we can downgrade now but unrelated to this PR so can done later.