build: fix iidfile content

[crazy-max]

fixes #420
fixes docker/build-push-action#461

output the image ID or digest with --iidfile flag and containerimage.digest response depending on the driver and output being used:

• docker driver : writes the image ID.
• docker driver with --push: writes the image digest.
• docker-container driver with --load: writes the image ID.
• docker-container driver with --push: writes the image digest.

[tonistiigi]

docker driver with --push: writes the image digest.

Is this the behavior that we want?

[crazy-max]

docker driver with --push: writes the image digest.

Is this the behavior that we want?

I see what you mean as the image is in docker store but also pushed to the registry. I agree for feature parity it might be better to keep the image ID for this case. Wonder if we could somehow provide the image digest in the exporter response (other key?) or another flag?

[crazy-max]

#980 will fix the imageID issue with the container driver. About image digest I think it's a misleading issue in our github action with the digest output. Should be called imageid instead and we should change the digest output behavior to only add image digest if image is effectively pushed. WDYT?

[tonistiigi]

Should be called imageid instead and we should change the digest output behavior to only add image digest if image is effectively pushed. WDYT?

Yes, separate outputs for the action. The digest is filled whenever it is available, not only with push.