stage_executor: allow images without layers

[flouthoc]

A build case where base is scratch and nothing else, then produce image without layer.

This matches with buildkit and docker.

What type of PR is this?

/kind api-change
/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake
/kind other

What this PR does / why we need it:

How to verify it

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

 stage_executor: allow images without layers

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: flouthoc

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [flouthoc]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[rhatdan]

LGTM

[TomSweeneyRedHat]

LGTM
But I would suggest changing the release note to:

When a build uses "scratch" for the image and only a label is added, a layer is now added to the image rather than creating an empty image.

[nalind]

If an image with no layers is a valid thing, then this is a bug in the logic that lists which images are in storage, and that's where this needs to be fixed. I'd suggest adding error context with fmt.Errorf() in layer_tree.go and going from there.

[flouthoc]

@nalind SGTM but one thing which bothers me is that when a single FROM scratch is created without any labels it creates a layer, but as soon as a label is added it does not creates any layer in the image. That part looked inconsistent.

If an image with no layers is a valid thing, then this is a bug in the logic

Yes this is which i needed to verify, checking on this.

[flouthoc]

Well it seems, docker/buildkit allows creating image without layers so we gotta do that do, and buildkit produces no layer even for FROM scratch , I'll modify this PR to make behavior of FROM scratch consistent with buildkit.

[flouthoc]

/hold

[TomSweeneyRedHat]

@flouthoc looks like you'll need to rebase.

[flouthoc]

@nalind I have modified this PR , and using this PR to make buildah's behavior consistent with docker and buildkit and meanwhile the original issue , i'm trying to fix that here: containers/common#1651

[flouthoc]

@nalind @containers/buildah-maintainers PTAL original issue containers/podman#19860 is closed in a different PR, following PR brings consistency with buildkit and docker

[nalind]

LGTM

[rhatdan]

/lgtm