builder: fix config leakage on shared parent stage

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

components/engine/builder/dockerfile/evaluator.go

@@ -220,7 +220,7 @@ func (s *dispatchState) beginStage(stageName string, image builder.Image) {
 	s.imageID = image.ImageID()
 
 	if image.RunConfig() != nil {
-		s.runConfig = image.RunConfig()
+		s.runConfig = copyRunConfig(image.RunConfig())
 	} else {
 		s.runConfig = &container.Config{}
 	}

components/engine/integration-cli/docker_cli_build_test.go

@@ -6234,6 +6234,28 @@ func (s *DockerSuite) TestBuildCopyFromResetScratch(c *check.C) {
 	c.Assert(strings.TrimSpace(res), checker.Equals, "")
 }
 
+func (s *DockerSuite) TestBuildMultiStageParentConfig(c *check.C) {
+	testRequires(c, DaemonIsLinux)
+
+	dockerfile := `
+		FROM busybox AS stage0
+		WORKDIR /foo
+		FROM stage0
+		WORKDIR sub1
+		FROM stage0
+		WORKDIR sub2
+		`
+	ctx := fakecontext.New(c, "",
+		fakecontext.WithDockerfile(dockerfile),
+	)
+	defer ctx.Close()
+
+	cli.BuildCmd(c, "build1", build.WithExternalBuildContext(ctx))
+
+	res := cli.InspectCmd(c, "build1", cli.Format(".Config.WorkingDir")).Combined()
+	c.Assert(strings.TrimSpace(res), checker.Equals, "/foo/sub2")
+}
+
 func (s *DockerSuite) TestBuildIntermediateTarget(c *check.C) {
 	dockerfile := `
 		FROM busybox AS build-env