build: set remote digest when pushed with docker driver

[crazy-max]

closes #975

Set containerimage.digest with correct digest when image is pushed with moby. If containerimage.digest already exists, its value will be saved to containerimage.config.digest for backward compatibility with #980.

repro: https://github.com/crazy-max/buildx-buildkit-tests/runs/5440498633?check_suite_focus=true#step:7:153

In follow-up create the imageid output in our GitHub Action and also fix digest output. See docker/build-push-action#569

Signed-off-by: CrazyMax [email protected]

[tonistiigi]

why this? Is this backward compatibility or smth? Why wouldn't ExporterImageConfigDigestKey be set already?

[crazy-max]

Why wouldn't ExporterImageConfigDigestKey be set already?

As you can see in https://github.com/crazy-max/buildx-buildkit-tests/runs/5458103650?check_suite_focus=true#step:7:152 (using latest stable buildx version), metadata only returns the following content when image is pushed with moby:

{
  "containerimage.digest": "sha256:1fd302965e74b804eccf7e6ab6ec1edcbd3dfa8b8371ebc7244492832c39dd81"
}

I think this is linked to the current implementation on moby: https://github.com/moby/moby/blob/327699c313f51f8062cf3affcb3746f56f3689c6/builder/builder-next/exporter/export.go#L176

[tonistiigi]

Ok, we should try to fix this in moby. To make sure we only do this for the old daemons lets only set ExporterImageConfigDigestKey when no such value exists and add a comment that this is for handling older daemons that don't return ExporterImageConfigDigestKey.