@@ -214,7 +214,8 @@ func (s *dispatchState) beginStage(stageName string, image builder.Image) {

 	s.imageID = image.ImageID()

 	if image.RunConfig() != nil {

-		s.runConfig = copyRunConfig(image.RunConfig()) // copy avoids referencing the same instance when 2 stages have the same base

+		// copy avoids referencing the same instance when 2 stages have the same base

+		s.runConfig = copyRunConfig(image.RunConfig())

 	} else {

 		s.runConfig = &container.Config{}

 	}

@@ -25,6 +25,7 @@ import (

 	"github.com/docker/docker/pkg/stringid"

 	"github.com/docker/docker/pkg/symlink"

 	"github.com/docker/docker/pkg/system"

+	"github.com/docker/go-connections/nat"

 	lcUser "github.com/opencontainers/runc/libcontainer/user"

 	"github.com/pkg/errors"

 )

@@ -385,14 +386,6 @@ func hashStringSlice(prefix string, slice []string) string {

 type runConfigModifier func(*container.Config)

-func copyRunConfig(runConfig *container.Config, modifiers ...runConfigModifier) *container.Config {

-	copy := *runConfig

-	for _, modifier := range modifiers {

-		modifier(&copy)

-	}

-	return &copy

-}

-

 func withCmd(cmd []string) runConfigModifier {

 	return func(runConfig *container.Config) {

 		runConfig.Cmd = cmd

@@ -438,6 +431,48 @@ func withEntrypointOverride(cmd []string, entrypoint []string) runConfigModifier

 	}

 }

+func copyRunConfig(runConfig *container.Config, modifiers ...runConfigModifier) *container.Config {

+	copy := *runConfig

+	copy.Cmd = copyStringSlice(runConfig.Cmd)

+	copy.Env = copyStringSlice(runConfig.Env)

+	copy.Entrypoint = copyStringSlice(runConfig.Entrypoint)

+	copy.OnBuild = copyStringSlice(runConfig.OnBuild)

+	copy.Shell = copyStringSlice(runConfig.Shell)

+

+	if copy.Volumes != nil {

+		copy.Volumes = make(map[string]struct{}, len(runConfig.Volumes))

+		for k, v := range runConfig.Volumes {

+			copy.Volumes[k] = v

+		}

+	}

+

+	if copy.ExposedPorts != nil {

+		copy.ExposedPorts = make(nat.PortSet, len(runConfig.ExposedPorts))

+		for k, v := range runConfig.ExposedPorts {

+			copy.ExposedPorts[k] = v

+		}

+	}

+

+	if copy.Labels != nil {

+		copy.Labels = make(map[string]string, len(runConfig.Labels))

+		for k, v := range runConfig.Labels {

+			copy.Labels[k] = v

+		}

+	}

+

+	for _, modifier := range modifiers {

+		modifier(&copy)

+	}

+	return &copy

+}

+

+func copyStringSlice(orig []string) []string {

+	if orig == nil {

+		return nil

+	}

+	return append([]string{}, orig...)

+}

+

 // getShell is a helper function which gets the right shell for prefixing the

 // shell-form of RUN, ENTRYPOINT and CMD instructions

 func getShell(c *container.Config, os string) []string {

@@ -14,6 +14,7 @@ import (

 	"github.com/docker/docker/builder/remotecontext"

 	"github.com/docker/docker/pkg/archive"

 	"github.com/docker/docker/pkg/idtools"

+	"github.com/docker/go-connections/nat"

 	"github.com/stretchr/testify/assert"

 	"github.com/stretchr/testify/require"

 )

@@ -133,6 +134,44 @@ func TestCopyRunConfig(t *testing.T) {

 }

+func fullMutableRunConfig() *container.Config {

+	return &container.Config{

+		Cmd: []string{"command", "arg1"},

+		Env: []string{"env1=foo", "env2=bar"},

+		ExposedPorts: nat.PortSet{

+			"1000/tcp": {},

+			"1001/tcp": {},

+		},

+		Volumes: map[string]struct{}{

+			"one": {},

+			"two": {},

+		},

+		Entrypoint: []string{"entry", "arg1"},

+		OnBuild:    []string{"first", "next"},

+		Labels: map[string]string{

+			"label1": "value1",

+			"label2": "value2",

+		},

+		Shell: []string{"shell", "-c"},

+	}

+}

+

+func TestDeepCopyRunConfig(t *testing.T) {

+	runConfig := fullMutableRunConfig()

+	copy := copyRunConfig(runConfig)

+	assert.Equal(t, fullMutableRunConfig(), copy)

+

+	copy.Cmd[1] = "arg2"

+	copy.Env[1] = "env2=new"

+	copy.ExposedPorts["10002"] = struct{}{}

+	copy.Volumes["three"] = struct{}{}

+	copy.Entrypoint[1] = "arg2"

+	copy.OnBuild[0] = "start"

+	copy.Labels["label3"] = "value3"

+	copy.Shell[0] = "sh"

+	assert.Equal(t, fullMutableRunConfig(), runConfig)

+}

+

 func TestChownFlagParsing(t *testing.T) {

 	testFiles := map[string]string{

 		"passwd": `root:x:0:0::/bin:/bin/false

@@ -6,13 +6,16 @@ import (

 	"context"

 	"encoding/json"

 	"io"

+	"io/ioutil"

 	"strings"

 	"testing"

 	"github.com/docker/docker/api/types"

 	"github.com/docker/docker/api/types/filters"

+	"github.com/docker/docker/integration-cli/cli/build/fakecontext"

 	"github.com/docker/docker/integration/util/request"

 	"github.com/docker/docker/pkg/jsonmessage"

+	"github.com/stretchr/testify/assert"

 	"github.com/stretchr/testify/require"

 )

@@ -129,3 +132,40 @@ func buildContainerIdsFilter(buildOutput io.Reader) (filters.Args, error) {

 		}

 	}

 }

+

+func TestBuildMultiStageParentConfig(t *testing.T) {

+	dockerfile := `

+		FROM busybox AS stage0

+		ENV WHO=parent

+		WORKDIR /foo

+

+		FROM stage0

+		ENV WHO=sibling1

+		WORKDIR sub1

+

+		FROM stage0

+		WORKDIR sub2

+	`

+	ctx := context.Background()

+	source := fakecontext.New(t, "", fakecontext.WithDockerfile(dockerfile))

+	defer source.Close()

+

+	apiclient := testEnv.APIClient()

+	resp, err := apiclient.ImageBuild(ctx,

+		source.AsTarReader(t),

+		types.ImageBuildOptions{

+			Remove:      true,

+			ForceRemove: true,

+			Tags:        []string{"build1"},

+		})

+	require.NoError(t, err)

+	_, err = io.Copy(ioutil.Discard, resp.Body)

+	resp.Body.Close()

+	require.NoError(t, err)

+

+	image, _, err := apiclient.ImageInspectWithRaw(ctx, "build1")

+	require.NoError(t, err)

+

+	assert.Equal(t, "/foo/sub2", image.Config.WorkingDir)

+	assert.Contains(t, image.Config.Env, "WHO=parent")

+}