dockerfile: add --unpack flag to ADD

[tonistiigi]

Currently, the default behavior is to extract archives from local paths and keep original archive on URL paths.

This keeps the default but allows setting explicit --unpack=bool flag for the opposite behavior.

Fixes #4482

[fenollp]

Hello! Looks like the post-merge CI failed and a new labs image was not pushed: https://github.com/moby/buildkit/actions/runs/15249959528/job/42885256145

Here shows a 429: https://github.com/moby/buildkit/actions/runs/15249959520/job/42884278789#step:3:1654

Please push a labs tag 🙏

[crazy-max]

Hello! Looks like the post-merge CI failed and a new labs image was not pushed: https://github.com/moby/buildkit/actions/runs/15249959528/job/42885256145

Here shows a 429: https://github.com/moby/buildkit/actions/runs/15249959520/job/42884278789#step:3:1654

Please push a labs tag 🙏

Ah yes this is related to GitHub infra issue: https://www.githubstatus.com/incidents/d0nm3xcdc5jw
Will re-run the workflows

[tonistiigi]

Please push a labs tag

FYI, this is not labs-only. @AkihiroSuda should it? or is it obvious enough?

[AkihiroSuda]

Seems safe to graduate from labs at birth