Issue #439 double quotes in ARG value

* Strip out double quotes enclosing ARG value after parsing
dockerfile

integration/dockerfiles/Dockerfile_test_arg_multi_with_quotes

@@ -0,0 +1,12 @@
+ARG FILE_NAME="myFile"
+
+FROM busybox:latest AS builder
+ARG FILE_NAME
+
+RUN echo $FILE_NAME && touch /$FILE_NAME.txt && stat /$FILE_NAME.txt;
+
+FROM busybox:latest
+ARG FILE_NAME
+
+RUN echo $FILE_NAME && touch /$FILE_NAME.txt && stat /$FILE_NAME.txt;
+COPY --from=builder /$FILE_NAME.txt /

pkg/dockerfile/dockerfile.go

@@ -111,9 +111,33 @@ func Parse(b []byte) ([]instructions.Stage, []instructions.ArgCommand, error) {
 	if err != nil {
 		return nil, nil, err
 	}
+
+	metaArgs = stripEnclosingDoubleQuotes(metaArgs)
+
 	return stages, metaArgs, nil
 }
 
+// stripEnclosingDoubleQuotes removes double quotes enclosing the value of each instructions.ArgCommand in a slice
+func stripEnclosingDoubleQuotes(metaArgs []instructions.ArgCommand) []instructions.ArgCommand {
+	for i := range metaArgs {
+		arg := metaArgs[i]
+		v := arg.Value
+		if v != nil {
+			val := *v
+			if val[0] == '"' {
+				val = val[1:]
+			}
+
+			if val[len(val)-1] == '"' {
+				val = val[:len(val)-1]
+			}
+			arg.Value = &val
+			metaArgs[i] = arg
+		}
+	}
+	return metaArgs
+}
+
 // targetStage returns the index of the target stage kaniko is trying to build
 func targetStage(stages []instructions.Stage, target string) (int, error) {
 	if target == "" {

pkg/dockerfile/dockerfile_test.go

@@ -17,13 +17,64 @@ limitations under the License.
 package dockerfile
 
 import (
+	"io/ioutil"
+	"os"
 	"strconv"
 	"testing"
 
+	"github.com/GoogleContainerTools/kaniko/pkg/config"
 	"github.com/GoogleContainerTools/kaniko/testutil"
 	"github.com/moby/buildkit/frontend/dockerfile/instructions"
 )
 
+func TestStagesArgValueWithQuotes(t *testing.T) {
+	dockerfile := `
+	ARG IMAGE="ubuntu:16.04"
+	FROM ${IMAGE}
+	RUN echo hi > /hi
+	
+	FROM scratch AS second
+	COPY --from=0 /hi /hi2
+	
+	FROM scratch
+	COPY --from=second /hi2 /hi3
+	`
+	tmpfile, err := ioutil.TempFile("", "Dockerfile.test")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer os.Remove(tmpfile.Name())
+
+	if _, err := tmpfile.Write([]byte(dockerfile)); err != nil {
+		t.Fatal(err)
+	}
+	if err := tmpfile.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	stages, err := Stages(&config.KanikoOptions{DockerfilePath: tmpfile.Name()})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(stages) == 0 {
+		t.Fatal("length of stages expected to be greater than zero, but was zero")
+
+	}
+
+	if len(stages[0].MetaArgs) == 0 {
+		t.Fatal("length of stage[0] meta args expected to be greater than zero, but was zero")
+	}
+
+	expectedVal := "ubuntu:16.04"
+
+	arg := stages[0].MetaArgs[0]
+	if arg.ValueString() != expectedVal {
+		t.Fatalf("expected stages[0].MetaArgs[0] val to be %s but was %s", expectedVal, arg.ValueString())
+	}
+}
+
 func Test_resolveStages(t *testing.T) {
 	dockerfile := `
 	FROM scratch