dockerfile: detect empty images early

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

frontend/dockerfile/dockerfile2llb/convert.go

@@ -156,6 +156,7 @@ func Dockerfile2LLB(ctx context.Context, dt []byte, opt ConvertOpt) (*llb.State,
 						return err
 					}
 					d.stage.BaseName = reference.TagNameOnly(ref).String()
+					var isScratch bool
 					if metaResolver != nil && reachable {
 						dgst, dt, err := metaResolver.ResolveImageConfig(ctx, d.stage.BaseName)
 						if err == nil { // handle the error while builder is actually running
@@ -173,9 +174,16 @@ func Dockerfile2LLB(ctx context.Context, dt []byte, opt ConvertOpt) (*llb.State,
 							}
 							d.stage.BaseName = ref.String()
 							_ = ref
+							if len(img.RootFS.DiffIDs) == 0 {
+								isScratch = true
 							}
 						}
+					}
+					if isScratch {
+						d.state = llb.Scratch()
+					} else {
 						d.state = llb.Image(d.stage.BaseName, dfCmd(d.stage.SourceCode))
+					}
 					return nil
 				})
 			}(i, d)

solver/llbsolver/bridge.go

@@ -88,10 +88,12 @@ func (b *llbBridge) Solve(ctx context.Context, req frontend.SolveRequest) (res s
 		if !ok {
 			return nil, nil, errors.Errorf("invalid reference for exporting: %T", res.Sys())
 		}
+		if wr.ImmutableRef != nil {
 			if err := wr.ImmutableRef.Finalize(ctx); err != nil {
 				return nil, nil, err
 			}
 		}
+	}
 	return
 }