@@ -187,20 +187,16 @@ func dispatchCopy(b *Builder, args []string, attributes map[string]bool, origina

 		return err

 	}

-	var contextID *int

+	var im *imageMount

 	if flFrom.IsUsed() {

 		var err error

-		context, err := strconv.Atoi(flFrom.Value)

+		im, err = b.imageContexts.get(flFrom.Value)

 		if err != nil {

-			return errors.Wrap(err, "from expects an integer value corresponding to the context number")

-		}

-		if err := b.imageContexts.validate(context); err != nil {

 			return err

 		}

-		contextID = &context

 	}

-	return b.runContextCommand(args, false, false, "COPY", contextID)

+	return b.runContextCommand(args, false, false, "COPY", im)

 }

 // FROM imagename

@@ -208,7 +204,13 @@ func dispatchCopy(b *Builder, args []string, attributes map[string]bool, origina

 // This sets the image the dockerfile will build on top of.

 //

 func from(b *Builder, args []string, attributes map[string]bool, original string) error {

-	if len(args) != 1 {

+	ctxName := ""

+	if len(args) == 3 && strings.EqualFold(args[1], "as") {

+		ctxName = strings.ToLower(args[2])

+		if ok, _ := regexp.MatchString("^[a-z][a-z0-9-_\\.]*$", ctxName); !ok {

+			return errors.Errorf("invalid name for build stage: %q, name can't start with a number or contain symbols", ctxName)

+		}

+	} else if len(args) != 1 {

 		return errExactlyOneArgument("FROM")

 	}

@@ -221,29 +223,32 @@ func from(b *Builder, args []string, attributes map[string]bool, original string

 	var image builder.Image

 	b.resetImageCache()

-	b.imageContexts.new()

+	if _, err := b.imageContexts.new(ctxName, true); err != nil {

+		return err

+	}

-	// Windows cannot support a container with no base image.

-	if name == api.NoBaseImageSpecifier {

-		if runtime.GOOS == "windows" {

-			return errors.New("Windows does not support FROM scratch")

+	if im, ok := b.imageContexts.byName[name]; ok {

+		if len(im.ImageID()) > 0 {

+			image = im

 		}

-		b.image = ""

-		b.noBaseImage = true

 	} else {

-		// TODO: don't use `name`, instead resolve it to a digest

-		if !b.options.PullParent {

-			image, _ = b.docker.GetImageOnBuild(name)

-			// TODO: shouldn't we error out if error is different from "not found" ?

-		}

-		if image == nil {

+		// Windows cannot support a container with no base image.

+		if name == api.NoBaseImageSpecifier {

+			if runtime.GOOS == "windows" {

+				return errors.New("Windows does not support FROM scratch")

+			}

+			b.image = ""

+			b.noBaseImage = true

+		} else {

 			var err error

-			image, err = b.docker.PullOnBuild(b.clientCtx, name, b.options.AuthConfigs, b.Output)

+			image, err = pullOrGetImage(b, name)

 			if err != nil {

 				return err

 			}

 		}

-		b.imageContexts.update(image.ImageID())

+	}

+	if image != nil {

+		b.imageContexts.update(image.ImageID(), image.RunConfig())

 	}

 	b.from = image

@@ -831,3 +836,33 @@ func getShell(c *container.Config) []string {

 	}

 	return append([]string{}, c.Shell[:]...)

 }

+

+// mountByRef creates an imageMount from a reference. pulling the image if needed.

+func mountByRef(b *Builder, name string) (*imageMount, error) {

+	image, err := pullOrGetImage(b, name)

+	if err != nil {

+		return nil, err

+	}

+	im, err := b.imageContexts.new("", false)

+	if err != nil {

+		return nil, err

+	}

+	im.id = image.ImageID()

+	return im, nil

+}

+

+func pullOrGetImage(b *Builder, name string) (builder.Image, error) {

+	var image builder.Image

+	if !b.options.PullParent {

+		image, _ = b.docker.GetImageOnBuild(name)

+		// TODO: shouldn't we error out if error is different from "not found" ?

+	}

+	if image == nil {

+		var err error

+		image, err = b.docker.PullOnBuild(b.clientCtx, name, b.options.AuthConfigs, b.Output)

+		if err != nil {

+			return nil, err

+		}

+	}

+	return image, nil

+}

@@ -1,9 +1,12 @@

 package dockerfile

 import (

+	"strconv"

+	"strings"

 	"sync"

 	"github.com/Sirupsen/logrus"

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

 	"github.com/docker/docker/builder"

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

 	"github.com/pkg/errors"

@@ -12,23 +15,32 @@ import (

 // imageContexts is a helper for stacking up built image rootfs and reusing

 // them as contexts

 type imageContexts struct {

-	b     *Builder

-	list  []*imageMount

-	cache *pathCache

+	b      *Builder

+	list   []*imageMount

+	byName map[string]*imageMount

+	cache  *pathCache

 }

-type imageMount struct {

-	id      string

-	ctx     builder.Context

-	release func() error

-}

-

-func (ic *imageContexts) new() {

-	ic.list = append(ic.list, &imageMount{})

+func (ic *imageContexts) new(name string, increment bool) (*imageMount, error) {

+	im := &imageMount{ic: ic}

+	if len(name) > 0 {

+		if ic.byName == nil {

+			ic.byName = make(map[string]*imageMount)

+		}

+		if _, ok := ic.byName[name]; ok {

+			return nil, errors.Errorf("duplicate name %s", name)

+		}

+		ic.byName[name] = im

+	}

+	if increment {

+		ic.list = append(ic.list, im)

+	}

+	return im, nil

 }

-func (ic *imageContexts) update(imageID string) {

+func (ic *imageContexts) update(imageID string, runConfig *container.Config) {

 	ic.list[len(ic.list)-1].id = imageID

+	ic.list[len(ic.list)-1].runConfig = runConfig

 }

 func (ic *imageContexts) validate(i int) error {

@@ -42,16 +54,72 @@ func (ic *imageContexts) validate(i int) error {

 	return nil

 }

-func (ic *imageContexts) context(i int) (builder.Context, error) {

-	if err := ic.validate(i); err != nil {

-		return nil, err

+func (ic *imageContexts) get(indexOrName string) (*imageMount, error) {

+	index, err := strconv.Atoi(indexOrName)

+	if err == nil {

+		if err := ic.validate(index); err != nil {

+			return nil, err

+		}

+		return ic.list[index], nil

+	}

+	if im, ok := ic.byName[strings.ToLower(indexOrName)]; ok {

+		return im, nil

+	}

+	im, err := mountByRef(ic.b, indexOrName)

+	if err != nil {

+		return nil, errors.Wrapf(err, "invalid from flag value %s", indexOrName)

+	}

+	return im, nil

+}

+

+func (ic *imageContexts) unmount() (retErr error) {

+	for _, im := range ic.list {

+		if err := im.unmount(); err != nil {

+			logrus.Error(err)

+			retErr = err

+		}

+	}

+	for _, im := range ic.byName {

+		if err := im.unmount(); err != nil {

+			logrus.Error(err)

+			retErr = err

+		}

+	}

+	return

+}

+

+func (ic *imageContexts) getCache(id, path string) (interface{}, bool) {

+	if ic.cache != nil {

+		if id == "" {

+			return nil, false

+		}

+		return ic.cache.get(id + path)

+	}

+	return nil, false

+}

+

+func (ic *imageContexts) setCache(id, path string, v interface{}) {

+	if ic.cache != nil {

+		ic.cache.set(id+path, v)

 	}

-	im := ic.list[i]

+}

+

+// imageMount is a reference for getting access to a buildcontext that is backed

+// by an existing image

+type imageMount struct {

+	id        string

+	ctx       builder.Context

+	release   func() error

+	ic        *imageContexts

+	runConfig *container.Config

+}

+

+func (im *imageMount) context() (builder.Context, error) {

 	if im.ctx == nil {

 		if im.id == "" {

 			return nil, errors.Errorf("could not copy from empty context")

 		}

-		p, release, err := ic.b.docker.MountImage(im.id)

+		p, release, err := im.ic.b.docker.MountImage(im.id)

 		if err != nil {

 			return nil, errors.Wrapf(err, "failed to mount %s", im.id)

 		}

@@ -59,40 +127,27 @@ func (ic *imageContexts) context(i int) (builder.Context, error) {

 		if err != nil {

 			return nil, errors.Wrapf(err, "failed to create lazycontext for %s", p)

 		}

-		logrus.Debugf("mounted image: %s %s", im.id, p)

 		im.release = release

 		im.ctx = ctx

 	}

 	return im.ctx, nil

 }

-func (ic *imageContexts) unmount() (retErr error) {

-	for _, im := range ic.list {

-		if im.release != nil {

-			if err := im.release(); err != nil {

-				logrus.Error(errors.Wrapf(err, "failed to unmount previous build image"))

-				retErr = err

-			}

+func (im *imageMount) unmount() error {

+	if im.release != nil {

+		if err := im.release(); err != nil {

+			return errors.Wrapf(err, "failed to unmount previous build image %s", im.id)

 		}

+		im.release = nil

 	}

-	return

+	return nil

 }

-func (ic *imageContexts) getCache(i int, path string) (interface{}, bool) {

-	if ic.cache != nil {

-		im := ic.list[i]

-		if im.id == "" {

-			return nil, false

-		}

-		return ic.cache.get(im.id + path)

-	}

-	return nil, false

+func (im *imageMount) ImageID() string {

+	return im.id

 }

-

-func (ic *imageContexts) setCache(i int, path string, v interface{}) {

-	if ic.cache != nil {

-		ic.cache.set(ic.list[i].id+path, v)

-	}

+func (im *imageMount) RunConfig() *container.Config {

+	return im.runConfig

 }

 type pathCache struct {

@@ -85,7 +85,7 @@ func (b *Builder) commit(id string, autoCmd strslice.StrSlice, comment string) e

 	}

 	b.image = imageID

-	b.imageContexts.update(imageID)

+	b.imageContexts.update(imageID, &autoConfig)

 	return nil

 }

@@ -94,7 +94,7 @@ type copyInfo struct {

 	decompress bool

 }

-func (b *Builder) runContextCommand(args []string, allowRemote bool, allowLocalDecompression bool, cmdName string, contextID *int) error {

+func (b *Builder) runContextCommand(args []string, allowRemote bool, allowLocalDecompression bool, cmdName string, imageSource *imageMount) error {

 	if len(args) < 2 {

 		return fmt.Errorf("Invalid %s format - at least two arguments required", cmdName)

 	}

@@ -128,7 +128,7 @@ func (b *Builder) runContextCommand(args []string, allowRemote bool, allowLocalD

 			continue

 		}

 		// not a URL

-		subInfos, err := b.calcCopyInfo(cmdName, orig, allowLocalDecompression, true, contextID)

+		subInfos, err := b.calcCopyInfo(cmdName, orig, allowLocalDecompression, true, imageSource)

 		if err != nil {

 			return err

 		}

@@ -298,13 +298,13 @@ func (b *Builder) download(srcURL string) (fi builder.FileInfo, err error) {

 	return &builder.HashedFileInfo{FileInfo: builder.PathFileInfo{FileInfo: tmpFileSt, FilePath: tmpFileName}, FileHash: hash}, nil

 }

-func (b *Builder) calcCopyInfo(cmdName, origPath string, allowLocalDecompression, allowWildcards bool, contextID *int) ([]copyInfo, error) {

+func (b *Builder) calcCopyInfo(cmdName, origPath string, allowLocalDecompression, allowWildcards bool, imageSource *imageMount) ([]copyInfo, error) {

 	// Work in daemon-specific OS filepath semantics

 	origPath = filepath.FromSlash(origPath)

 	// validate windows paths from other images

-	if contextID != nil && runtime.GOOS == "windows" {

+	if imageSource != nil && runtime.GOOS == "windows" {

 		forbid := regexp.MustCompile("(?i)^" + string(os.PathSeparator) + "?(windows(" + string(os.PathSeparator) + ".+)?)?$")

 		if p := filepath.Clean(origPath); p == "." || forbid.MatchString(p) {

 			return nil, errors.Errorf("copy from %s is not allowed on windows", origPath)

@@ -318,8 +318,8 @@ func (b *Builder) calcCopyInfo(cmdName, origPath string, allowLocalDecompression

 	context := b.context

 	var err error

-	if contextID != nil {

-		context, err = b.imageContexts.context(*contextID)

+	if imageSource != nil {

+		context, err = imageSource.context()

 		if err != nil {

 			return nil, err

 		}

@@ -346,7 +346,7 @@ func (b *Builder) calcCopyInfo(cmdName, origPath string, allowLocalDecompression

 			// Note we set allowWildcards to false in case the name has

 			// a * in it

-			subInfos, err := b.calcCopyInfo(cmdName, path, allowLocalDecompression, false, contextID)

+			subInfos, err := b.calcCopyInfo(cmdName, path, allowLocalDecompression, false, imageSource)

 			if err != nil {

 				return err

 			}

@@ -370,9 +370,9 @@ func (b *Builder) calcCopyInfo(cmdName, origPath string, allowLocalDecompression

 	if !handleHash {

 		return copyInfos, nil

 	}

-	if contextID != nil {

+	if imageSource != nil {

 		// fast-cache based on imageID

-		if h, ok := b.imageContexts.getCache(*contextID, origPath); ok {

+		if h, ok := b.imageContexts.getCache(imageSource.id, origPath); ok {

 			hfi.SetHash(h.(string))

 			return copyInfos, nil

 		}

@@ -401,8 +401,8 @@ func (b *Builder) calcCopyInfo(cmdName, origPath string, allowLocalDecompression

 	hasher := sha256.New()

 	hasher.Write([]byte(strings.Join(subfiles, ",")))

 	hfi.SetHash("dir:" + hex.EncodeToString(hasher.Sum(nil)))

-	if contextID != nil {

-		b.imageContexts.setCache(*contextID, origPath, hfi.Hash())

+	if imageSource != nil {

+		b.imageContexts.setCache(imageSource.id, origPath, hfi.Hash())

 	}

 	return copyInfos, nil

@@ -497,7 +497,7 @@ func (b *Builder) probeCache() (bool, error) {

 	fmt.Fprint(b.Stdout, " ---> Using cache\n")

 	logrus.Debugf("[BUILDER] Use cached version: %s", b.runConfig.Cmd)

 	b.image = string(cache)

-	b.imageContexts.update(b.image)

+	b.imageContexts.update(b.image, b.runConfig)

 	return true, nil

 }

@@ -106,7 +106,7 @@ func init() {

 		command.Entrypoint:  parseMaybeJSON,

 		command.Env:         parseEnv,

 		command.Expose:      parseStringsWhitespaceDelimited,

-		command.From:        parseString,

+		command.From:        parseStringsWhitespaceDelimited,

 		command.Healthcheck: parseHealthConfig,

 		command.Label:       parseLabel,

 		command.Maintainer:  parseString,

@@ -5752,7 +5752,7 @@ func (s *DockerSuite) TestBuildContChar(c *check.C) {

 func (s *DockerSuite) TestBuildCopyFromPreviousRootFS(c *check.C) {

 	dockerfile := `

-		FROM busybox

+		FROM busybox AS first

 		COPY foo bar

 		FROM busybox

     %s

@@ -5762,7 +5762,8 @@ func (s *DockerSuite) TestBuildCopyFromPreviousRootFS(c *check.C) {

     COPY bar /

     COPY --from=1 baz sub/

     COPY --from=0 bar baz

-    COPY --from=0 bar bay`

+    COPY --from=first bar bay`

+

 	ctx := fakeContext(c, fmt.Sprintf(dockerfile, ""), map[string]string{

 		"Dockerfile": dockerfile,

 		"foo":        "abc",

@@ -5830,7 +5831,7 @@ func (s *DockerSuite) TestBuildCopyFromPreviousRootFSErrors(c *check.C) {

 	buildImage("build1", withExternalBuildContext(ctx)).Assert(c, icmd.Expected{

 		ExitCode: 1,

-		Err:      "from expects an integer value corresponding to the context number",

+		Err:      "invalid from flag value foo",

 	})

 	dockerfile = `

@@ -5847,6 +5848,36 @@ func (s *DockerSuite) TestBuildCopyFromPreviousRootFSErrors(c *check.C) {

 		ExitCode: 1,

 		Err:      "invalid from flag value 0 refers current build block",

 	})

+

+	dockerfile = `

+		FROM busybox AS foo

+		COPY --from=bar foo bar`

+

+	ctx = fakeContext(c, dockerfile, map[string]string{

+		"Dockerfile": dockerfile,

+		"foo":        "abc",

+	})

+	defer ctx.Close()

+

+	buildImage("build1", withExternalBuildContext(ctx)).Assert(c, icmd.Expected{

+		ExitCode: 1,

+		Err:      "invalid from flag value bar",

+	})

+

+	dockerfile = `

+		FROM busybox AS 1

+		COPY --from=1 foo bar`

+

+	ctx = fakeContext(c, dockerfile, map[string]string{

+		"Dockerfile": dockerfile,

+		"foo":        "abc",

+	})

+	defer ctx.Close()

+

+	buildImage("build1", withExternalBuildContext(ctx)).Assert(c, icmd.Expected{

+		ExitCode: 1,

+		Err:      "invalid name for build stage",

+	})

 }

 func (s *DockerSuite) TestBuildCopyFromPreviousFrom(c *check.C) {

@@ -5863,9 +5894,9 @@ func (s *DockerSuite) TestBuildCopyFromPreviousFrom(c *check.C) {

 	result.Assert(c, icmd.Success)

 	dockerfile = `

-		FROM build1:latest

+		FROM build1:latest AS foo

     FROM busybox

-		COPY --from=0 bar /

+		COPY --from=foo bar /

 		COPY foo /`

 	ctx = fakeContext(c, dockerfile, map[string]string{

 		"Dockerfile": dockerfile,

@@ -5882,6 +5913,117 @@ func (s *DockerSuite) TestBuildCopyFromPreviousFrom(c *check.C) {

 	c.Assert(strings.TrimSpace(out), check.Equals, "def")

 }

+func (s *DockerSuite) TestBuildCopyFromImplicitFrom(c *check.C) {

+	dockerfile := `

+		FROM busybox

+		COPY --from=busybox /etc/passwd /mypasswd

+		RUN cmp /etc/passwd /mypasswd`

+

+	if DaemonIsWindows() {

+		dockerfile = `

+			FROM busybox

+			COPY --from=busybox License.txt foo`

+	}

+

+	ctx := fakeContext(c, dockerfile, map[string]string{

+		"Dockerfile": dockerfile,

+	})

+	defer ctx.Close()

+

+	result := buildImage("build1", withExternalBuildContext(ctx))

+	result.Assert(c, icmd.Success)

+

+	if DaemonIsWindows() {

+		out, _ := dockerCmd(c, "run", "build1", "cat", "License.txt")

+		c.Assert(len(out), checker.GreaterThan, 10)

+		out2, _ := dockerCmd(c, "run", "build1", "cat", "foo")

+		c.Assert(out, check.Equals, out2)

+	}

+}

+

+func (s *DockerRegistrySuite) TestBuildCopyFromImplicitPullingFrom(c *check.C) {

+	repoName := fmt.Sprintf("%v/dockercli/testf", privateRegistryURL)

+

+	dockerfile := `

+		FROM busybox

+		COPY foo bar`

+	ctx := fakeContext(c, dockerfile, map[string]string{

+		"Dockerfile": dockerfile,

+		"foo":        "abc",

+	})

+	defer ctx.Close()

+

+	result := buildImage(repoName, withExternalBuildContext(ctx))

+	result.Assert(c, icmd.Success)

+

+	dockerCmd(c, "push", repoName)

+	dockerCmd(c, "rmi", repoName)

+

+	dockerfile = `

+		FROM busybox

+		COPY --from=%s bar baz`

+

+	ctx = fakeContext(c, fmt.Sprintf(dockerfile, repoName), map[string]string{

+		"Dockerfile": dockerfile,

+	})

+	defer ctx.Close()

+

+	result = buildImage("build1", withExternalBuildContext(ctx))

+	result.Assert(c, icmd.Success)

+

+	dockerCmdWithResult("run", "build1", "cat", "baz").Assert(c, icmd.Expected{Out: "abc"})

+}

+

+func (s *DockerSuite) TestBuildFromPreviousBlock(c *check.C) {

+	dockerfile := `

+		FROM busybox as foo

+		COPY foo /

+		FROM foo as foo1

+		RUN echo 1 >> foo

+		FROM foo as foO2

+		RUN echo 2 >> foo

+		FROM foo

+		COPY --from=foo1 foo f1

+		COPY --from=FOo2 foo f2

+		` // foo2 case also tests that names are canse insensitive

+	ctx := fakeContext(c, dockerfile, map[string]string{

+		"Dockerfile": dockerfile,

+		"foo":        "bar",

+	})

+	defer ctx.Close()

+

+	result := buildImage("build1", withExternalBuildContext(ctx))

+	result.Assert(c, icmd.Success)

+

+	dockerCmdWithResult("run", "build1", "cat", "foo").Assert(c, icmd.Expected{Out: "bar"})

+

+	dockerCmdWithResult("run", "build1", "cat", "f1").Assert(c, icmd.Expected{Out: "bar1"})

+

+	dockerCmdWithResult("run", "build1", "cat", "f2").Assert(c, icmd.Expected{Out: "bar2"})

+}

+

+func (s *DockerTrustSuite) TestCopyFromTrustedBuild(c *check.C) {

+	img1 := s.setupTrustedImage(c, "trusted-build1")

+	img2 := s.setupTrustedImage(c, "trusted-build2")

+	dockerFile := fmt.Sprintf(`

+	FROM %s AS build-base

+	RUN echo ok > /foo

+	FROM %s

+	COPY --from=build-base foo bar`, img1, img2)

+

+	name := "testcopyfromtrustedbuild"

+

+	r := buildImage(name, trustedBuild, build.WithDockerfile(dockerFile))

+	r.Assert(c, icmd.Expected{

+		Out: fmt.Sprintf("FROM %s@sha", img1[:len(img1)-7]),

+	})

+	r.Assert(c, icmd.Expected{

+		Out: fmt.Sprintf("FROM %s@sha", img2[:len(img2)-7]),

+	})

+

+	dockerCmdWithResult("run", name, "cat", "bar").Assert(c, icmd.Expected{Out: "ok"})

+}

+

 // TestBuildOpaqueDirectory tests that a build succeeds which

 // creates opaque directories.

 // See https://github.com/docker/docker/issues/25244