@@ -189,15 +189,20 @@ func dispatchCopy(b *Builder, args []string, attributes map[string]bool, origina

 	var contextID *int

 	if flFrom.IsUsed() {

-		var err error

-		context, err := strconv.Atoi(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

+		flFrom.Value = strings.ToLower(flFrom.Value)

+		if context, ok := b.imageContexts.byName[flFrom.Value]; ok {

+			contextID = &context

+		} else {

+			var err error

+			context, err := strconv.Atoi(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

 		}

-		contextID = &context

 	}

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

@@ -208,7 +213,13 @@ 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")

 	}

@@ -221,7 +232,9 @@ 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); err != nil {

+		return err

+	}

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

 	if name == api.NoBaseImageSpecifier {

@@ -12,9 +12,10 @@ 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]int

+	cache  *pathCache

 }

 type imageMount struct {

@@ -23,8 +24,18 @@ type imageMount struct {

 	release func() error

 }

-func (ic *imageContexts) new() {

+func (ic *imageContexts) new(name string) error {

+	if len(name) > 0 {

+		if ic.byName == nil {

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

+		}

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

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

+		}

+		ic.byName[name] = len(ic.list)

+	}

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

+	return nil

 }

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

@@ -80,7 +80,7 @@ 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",

@@ -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 context 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,