stage_executor: allow images without layers

`Docker` and `Buildkit` creates no layers when only `FROM scratch` is
used as content for building image, buildah must do the same.

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

imagebuildah/stage_executor.go

@@ -1033,7 +1033,14 @@ func (s *StageExecutor) Execute(ctx context.Context, base string) (imgID string,
 			// squash the contents of the base image.  Whichever is
 			// the case, we need to commit() to create a new image.
 			logCommit(s.output, -1)
-			if imgID, ref, err = s.commit(ctx, s.getCreatedBy(nil, ""), false, s.output, s.executor.squash, lastStage); err != nil {
+			emptyLayer := false
+			if s.builder.FromImageID == "" && s.builder.FromImage == "" {
+				// If we only have a base image and its `scratch` and
+				// since there are no other childrens in this branch
+				// then treat this as an `empty_layer`.
+				emptyLayer = true
+			}
+			if imgID, ref, err = s.commit(ctx, s.getCreatedBy(nil, ""), emptyLayer, s.output, s.executor.squash, lastStage); err != nil {
 				return "", nil, fmt.Errorf("committing base container: %w", err)
 			}
 			// Generate build output if needed.

tests/bud.bats

@@ -89,6 +89,15 @@ _EOF
   validate_instance_compression "3" "$list" "arm64" "zstd"
 }
 
+@test "no layer should be created on scratch" {
+  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 "0" "layer should not exist"
+  run_buildah build --layers -t test -f $BUDFILES/from-scratch/Containerfile
+  run_buildah inspect -f '{{len .Docker.RootFS.DiffIDs}}' test
+  expect_output "0" "layer should not exist"
+}
+
 @test "bud: build push with --force-compression" {
   skip_if_no_podman
   local contextdir=${TEST_SCRATCH_DIR}/bud/platform