Log GitHub Actions runtime token access controls

[crazy-max]

fixes #433

This logs GitHub Actions runtime token access controls in the workflow so users can figure out if cache scope is readable/writable:

Example with this PR: https://github.com/docker/build-push-action/actions/runs/3283669905/jobs/5408685907#step:4:14

  [
    {
      "Scope": "refs/pull/707/merge",
      "Permission": 3
    },
    {
      "Scope": "refs/heads/master",
      "Permission": 1
    }
  ]

Push on master:

  [
    {
      "Scope": "refs/heads/master",
      "Permission": 3
    }
  ]

Permission:

• 1: read
• 2: write
• 3: read/write

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

[tonistiigi]

Should this be behind the debug option that is provided to workflows?

[crazy-max]

Should this be behind the debug option that is provided to workflows?

I'm not sure if that would be useful because ACTIONS_STEP_DEBUG secret needs to be set and therefore workflow re-triggered which can lead to inconvenient behavior.
Let's say you tagged a release and cache doesn't seem to work properly, you would have to trigger this workflow again with ACTIONS_STEP_DEBUG secret set to figure out what's going on and depending what this workflow does when releasing a project, this could overwrite release files, pushed image, etc... to just check scopes.