build: return imageID when loading without docker driver

[crazy-max]

fixes #420

follow-up #975 (comment)

repro https://github.com/crazy-max/buildx-buildkit-tests/runs/5374641648?check_suite_focus=true#step:7:2

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

[tonistiigi]

We should just use containerimage.config.digest always and if it is not set (eg. multi-platform) then the full digest as before.

No need to do anything special for --load as multi-platform don't load anyway.

Also need to make sure this doesn't break the github action and update if it may.

[crazy-max]

Also need to make sure this doesn't break the github action and update if it may.

It's fine for the GitHub Action with this behavior but as discussed we might create a new output named imageid that will contain the actual imageid and set digest output accordingly.

[tonistiigi]

It's fine for the GitHub Action with this behavior but as discussed we might create a new output named imageid that will contain the actual imageid and set digest output accordingly.

But what field does the action use atm? If it uses iidfile then it needs to be updated before the release.

[crazy-max]

Hum yeah atm digest output returns content of iidfile, so need to make changes for the action. Will do that.