Resolve env replacement for FROM command

integration/dockerfiles/Dockerfile_test_scratch

@@ -1,4 +1,5 @@
-FROM scratch
+ARG image
+FROM ${image}
 # First, make sure simple arg replacement works
 ARG file
 COPY $file /foo

integration/integration_test.go

@@ -98,6 +98,7 @@ func TestRun(t *testing.T) {
 		"Dockerfile_test_add":     {"file=context/foo"},
 		"Dockerfile_test_onbuild": {"file=/tmp/onbuild"},
 		"Dockerfile_test_scratch": {
+			"image=scratch",
 			"hello=hello-value",
 			"file=context/foo",
 			"file3=context/b*",

pkg/executor/executor.go

@@ -63,7 +63,10 @@ func DoBuild(dockerfilePath, srcContext, snapshotMode string, args []string) (na
 		return nil, nil, err
 	}
 	for index, stage := range stages {
-		baseImage := stage.BaseName
+		baseImage, err := util.ResolveEnvironmentReplacement(stage.BaseName, args, false)
+		if err != nil {
+			return nil, nil, err
+		}
 		finalStage := index == len(stages)-1
 		// Unpack file system to root
 		logrus.Infof("Unpacking filesystem of %s...", baseImage)