stage_executor: create layer if only empty layer is present

A build case where base is `scratch` and only a `label` is added then no
layer is created, its like tagging empty image, in such cases create a
layer.

Closes: containers/podman#19860

Signed-off-by: Aditya R <arajan@redhat.com>

imagebuildah/stage_executor.go

@@ -1410,6 +1410,13 @@ func (s *StageExecutor) Execute(ctx context.Context, base string) (imgID string,
 				}
 			}
 		} else {
+			emptyLayer := !s.stepRequiresLayer(step)
+			// If lastIntruction of this stage and there was only single step and base image is `scratch` then commit this layer
+			// instead of ignoring this as an `emptyLayer`. i.e do not create `emptyLayer` if only single layer is present in the
+			// entire build.
+			if len(stage.Node.Children) == 1 && lastInstruction && s.builder.FromImage == "" && s.builder.FromImageID == "" && emptyLayer {
+				emptyLayer = false
+			}
 			// We're not going to find any more cache hits, so we
 			// can stop looking for them.
 			checkForLayers = false
@@ -1420,7 +1427,7 @@ func (s *StageExecutor) Execute(ctx context.Context, base string) (imgID string,
 			// because at this point we want to save history for
 			// layers even if its a squashed build so that they
 			// can be part of build-cache.
-			imgID, ref, err = s.commit(ctx, s.getCreatedBy(node, addedContentSummary), !s.stepRequiresLayer(step), commitName, false)
+			imgID, ref, err = s.commit(ctx, s.getCreatedBy(node, addedContentSummary), emptyLayer, commitName, false)
 			if err != nil {
 				return "", nil, fmt.Errorf("committing container for step %+v: %w", *step, err)
 			}

tests/bud.bats

@@ -5782,6 +5782,12 @@ _EOF
   run_buildah rmi -f ${target}
 }
 
+@test "layer must be created on scratch and label" {
+  run_buildah build --layers --label "label1=value1" -t test -f $BUDFILES/from-scratch/Containerfile
+  run_buildah inspect -f '{{len .Docker.RootFS.DiffIDs}}' test
+  expect_output "1" "layer must exist"
+}
+
 
 @test "bud-from-relabel" {
   _prefetch alpine busybox