Skip the /kaniko directory when copying root (#2863)

This commit adds the skip option for otiai10.Copy to skip the /kaniko
directory when the root is being copied. The files under /kaniko dir
should be ignored and thus this shall not cause any loss of information.

fixes: GoogleContainerTools#2033

pkg/executor/copy_multistage_test.go

@@ -100,6 +100,7 @@ COPY --from=first copied/bam.txt output/`
 		testutil.CheckDeepEqual(t, 1, len(files))
 		testutil.CheckDeepEqual(t, files[0].Name(), "bam.txt")
 	})
+
 	t.Run("copy directory across multistage into a directory", func(t *testing.T) {
 		testDir, fn := setupMultistageTests(t)
 		defer fn()
@@ -135,6 +136,40 @@ COPY --from=first copied another`
 		//testutil.CheckDeepEqual(t, linkName, "bam.txt")
 	})
 
+	t.Run("copy root across multistage", func(t *testing.T) {
+		testDir, fn := setupMultistageTests(t)
+		defer fn()
+		dockerFile := `
+FROM scratch as first
+COPY foo copied
+ENV test test
+
+From scratch as second
+COPY --from=first / output/`
+		ioutil.WriteFile(filepath.Join(testDir, "workspace", "Dockerfile"), []byte(dockerFile), 0755)
+		opts := &config.KanikoOptions{
+			DockerfilePath: filepath.Join(testDir, "workspace", "Dockerfile"),
+			SrcContext:     filepath.Join(testDir, "workspace"),
+			SnapshotMode:   constants.SnapshotModeFull,
+		}
+		_, err := DoBuild(opts)
+		testutil.CheckNoError(t, err)
+
+		filesUnderRoot, err := ioutil.ReadDir(filepath.Join(testDir, "output/"))
+		if err != nil {
+			t.Fatal(err)
+		}
+		testutil.CheckDeepEqual(t, 3, len(filesUnderRoot))
+
+		files, err := ioutil.ReadDir(filepath.Join(testDir, "output/workspace/foo"))
+		if err != nil {
+			t.Fatal(err)
+		}
+		testutil.CheckDeepEqual(t, 2, len(files))
+		testutil.CheckDeepEqual(t, "bam.link", files[0].Name())
+		testutil.CheckDeepEqual(t, "bam.txt", files[1].Name())
+	})
+
 }
 
 func setupMultistageTests(t *testing.T) (string, func()) {