Add --target flag for multistage builds

README.md

@@ -275,6 +275,10 @@ Set this flag to strip timestamps out of the built image and make it reproducibl
 
 Set this flag as `--tarPath=<path>` to save the image as a tarball at path instead of pushing the image.
 
+#### --target
+
+Set this flag to indicate which build stage is the target build stage.
+
 ### Debug Image
 
 The kaniko executor image is based off of scratch and doesn't contain a shell.

cmd/executor/cmd/root.go

@@ -44,6 +44,7 @@ var (
 	tarPath                     string
 	singleSnapshot              bool
 	reproducible                bool
+	target                      string
 )
 
 func init() {
@@ -60,6 +61,7 @@ func init() {
 	RootCmd.PersistentFlags().StringVarP(&tarPath, "tarPath", "", "", "Path to save the image in as a tarball instead of pushing")
 	RootCmd.PersistentFlags().BoolVarP(&singleSnapshot, "single-snapshot", "", false, "Set this flag to take a single snapshot at the end of the build.")
 	RootCmd.PersistentFlags().BoolVarP(&reproducible, "reproducible", "", false, "Strip timestamps out of the image to make it reproducible")
+	RootCmd.PersistentFlags().StringVarP(&target, "target", "", "", " Set the target build stage to build")
 }
 
 var RootCmd = &cobra.Command{
@@ -92,6 +94,7 @@ var RootCmd = &cobra.Command{
 			Args:           buildArgs,
 			SingleSnapshot: singleSnapshot,
 			Reproducible:   reproducible,
+			Target:         target,
 		})
 		if err != nil {
 			logrus.Error(err)

integration/dockerfiles/Dockerfile_test_target

@@ -0,0 +1,10 @@
+FROM gcr.io/distroless/base:latest as base
+COPY . .
+
+FROM scratch as second
+ENV foopath context/foo
+COPY --from=0 $foopath context/b* /foo/
+
+FROM base
+ARG file
+COPY --from=second /foo $file

integration/integration_test.go

@@ -19,9 +19,6 @@ package integration
 import (
 	"encoding/json"
 	"fmt"
-	"github.com/GoogleContainerTools/kaniko/pkg/constants"
-	"github.com/google/go-containerregistry/pkg/name"
-	"github.com/google/go-containerregistry/pkg/v1/daemon"
 	"math"
 	"os"
 	"os/exec"
@@ -31,6 +28,10 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/GoogleContainerTools/kaniko/pkg/constants"
+	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/google/go-containerregistry/pkg/v1/daemon"
+
 	"github.com/GoogleContainerTools/kaniko/testutil"
 )
 
@@ -113,10 +114,15 @@ func TestRun(t *testing.T) {
 		"Dockerfile_test_multistage": {"file=/foo2"},
 	}
 
-	// Map for additional flags
-	additionalFlagsMap := map[string][]string{
+	// Map for additional docker flags
+	additionalDockerFlagsMap := map[string][]string{
+		"Dockerfile_test_target": {"--target=second"},
+	}
+	// Map for additional kaniko flags
+	additionalKanikoFlagsMap := map[string][]string{
 		"Dockerfile_test_add":     {"--single-snapshot"},
 		"Dockerfile_test_scratch": {"--single-snapshot"},
+		"Dockerfile_test_target":  {"--target=second"},
 	}
 
 	// TODO: remove test_user_run from this when https://github.com/GoogleContainerTools/container-diff/issues/237 is fixed
@@ -144,13 +150,14 @@ func TestRun(t *testing.T) {
 				buildArgs = append(buildArgs, arg)
 			}
 			// build docker image
+			additionalFlags := append(buildArgs, additionalDockerFlagsMap[dockerfile]...)
 			dockerImage := strings.ToLower(testRepo + dockerPrefix + dockerfile)
 			dockerCmd := exec.Command("docker",
 				append([]string{"build",
 					"-t", dockerImage,
 					"-f", path.Join(dockerfilesPath, dockerfile),
 					"."},
-					buildArgs...)...,
+					additionalFlags...)...,
 			)
 			RunCommand(dockerCmd, t)
 
@@ -173,7 +180,7 @@ func TestRun(t *testing.T) {
 			}
 
 			// build kaniko image
-			additionalFlags := append(buildArgs, additionalFlagsMap[dockerfile]...)
+			additionalFlags = append(buildArgs, additionalKanikoFlagsMap[dockerfile]...)
 			kanikoImage := strings.ToLower(testRepo + kanikoPrefix + dockerfile)
 			kanikoCmd := exec.Command("docker",
 				append([]string{"run",

pkg/dockerfile/dockerfile.go

@@ -18,13 +18,15 @@ package dockerfile
 
 import (
 	"bytes"
+	"fmt"
+	"path/filepath"
+	"strconv"
+	"strings"
+
 	"github.com/GoogleContainerTools/kaniko/pkg/constants"
 	"github.com/GoogleContainerTools/kaniko/pkg/util"
 	"github.com/docker/docker/builder/dockerfile/instructions"
 	"github.com/docker/docker/builder/dockerfile/parser"
-	"path/filepath"
-	"strconv"
-	"strings"
 )
 
 // Parse parses the contents of a Dockerfile and returns a list of commands
@@ -40,6 +42,15 @@ func Parse(b []byte) ([]instructions.Stage, error) {
 	return stages, err
 }
 
+func Validate(stages []instructions.Stage, target string) error {
+	for _, stage := range stages {
+		if stage.Name == target {
+			return nil
+		}
+	}
+	return fmt.Errorf("%s is not a valid target build stage", target)
+}
+
 // ResolveStages resolves any calls to previous stages with names to indices
 // Ex. --from=second_stage should be --from=1 for easier processing later on
 func ResolveStages(stages []instructions.Stage) {

pkg/dockerfile/dockerfile_test.go

@@ -18,13 +18,14 @@ package dockerfile
 
 import (
 	"fmt"
-	"github.com/GoogleContainerTools/kaniko/testutil"
-	"github.com/docker/docker/builder/dockerfile/instructions"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strconv"
 	"testing"
+
+	"github.com/GoogleContainerTools/kaniko/testutil"
+	"github.com/docker/docker/builder/dockerfile/instructions"
 )
 
 func Test_ResolveStages(t *testing.T) {
@@ -55,6 +56,45 @@ func Test_ResolveStages(t *testing.T) {
 	}
 }
 
+func Test_Validate(t *testing.T) {
+	dockerfile := `
+	FROM scratch
+	RUN echo hi > /hi
+	
+	FROM scratch AS second
+	COPY --from=0 /hi /hi2
+	
+	FROM scratch
+	COPY --from=second /hi2 /hi3
+	`
+	stages, err := Parse([]byte(dockerfile))
+	if err != nil {
+		t.Fatal(err)
+	}
+	tests := []struct {
+		name      string
+		target    string
+		shouldErr bool
+	}{
+		{
+			name:      "test valid target",
+			target:    "second",
+			shouldErr: false,
+		},
+		{
+			name:      "test invalid target",
+			target:    "invalid",
+			shouldErr: true,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			actualErr := Validate(stages, test.target)
+			testutil.CheckError(t, test.shouldErr, actualErr)
+		})
+	}
+}
+
 func Test_Dependencies(t *testing.T) {
 	testDir, err := ioutil.TempDir("", "")
 	if err != nil {

pkg/executor/executor.go

@@ -52,6 +52,7 @@ type KanikoBuildArgs struct {
 	Args           []string
 	SingleSnapshot bool
 	Reproducible   bool
+	Target         string
 }
 
 func DoBuild(k KanikoBuildArgs) (v1.Image, error) {
@@ -65,14 +66,17 @@ func DoBuild(k KanikoBuildArgs) (v1.Image, error) {
 	if err != nil {
 		return nil, err
 	}
+	if err := dockerfile.Validate(stages, k.Target); err != nil {
+		return nil, err
+	}
 	dockerfile.ResolveStages(stages)
 
 	hasher, err := getHasher(k.SnapshotMode)
 	if err != nil {
 		return nil, err
 	}
 	for index, stage := range stages {
-		finalStage := index == len(stages)-1
+		finalStage := (index == len(stages)-1) || (k.Target == stage.Name)
 		// Unpack file system to root
 		sourceImage, err := util.RetrieveSourceImage(index, k.Args, stages)
 		if err != nil {

pkg/util/image_util.go

@@ -72,6 +72,7 @@ func tarballImage(index int) (v1.Image, error) {
 }
 
 func remoteImage(image string) (v1.Image, error) {
+	logrus.Infof("Downloading base image %s", image)
 	ref, err := name.ParseReference(image, name.WeakValidation)
 	if err != nil {
 		return nil, err