Use pax tar format (GoogleContainerTools#1809)

* Use PAX tar format

* Add test case

pkg/util/tar_util.go

@@ -97,6 +97,8 @@ func (t *Tar) AddFileToTar(p string) error {
 	// this makes this layer unnecessarily differ from a cached layer which does contain this information
 	hdr.Uname = ""
 	hdr.Gname = ""
+	// use PAX format to preserve accurate mtime (match Docker behavior)
+	hdr.Format = tar.FormatPAX
 
 	hardlink, linkDst := t.checkHardlink(p, i)
 	if hardlink {

pkg/util/tar_util_test.go

@@ -17,12 +17,15 @@ limitations under the License.
 package util
 
 import (
+	"archive/tar"
+	"bytes"
 	"compress/gzip"
 	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"testing"
+	"time"
 
 	"github.com/GoogleContainerTools/kaniko/testutil"
 )
@@ -57,6 +60,39 @@ func Test_IsLocalTarArchive(t *testing.T) {
 	}
 }
 
+func Test_AddFileToTar(t *testing.T) {
+	testDir, err := ioutil.TempDir("", "")
+    if err != nil {
+        t.Fatalf("err setting up temp dir: %v", err)
+    }
+    defer os.RemoveAll(testDir)
+
+	path := filepath.Join(testDir, regularFiles[0])
+	if err := ioutil.WriteFile(path, []byte("hello"), os.ModePerm); err != nil {
+		t.Fatal(err)
+	}
+	// use a pre-determined time with non-zero microseconds to avoid flakiness
+	mtime := time.UnixMicro(1635533172891395)
+	if err := os.Chtimes(path, mtime, mtime); err != nil {
+		t.Fatal(err)
+	}
+
+	buf := new(bytes.Buffer)
+	tarw := NewTar(buf)
+	if err := tarw.AddFileToTar(path); err != nil {
+		t.Fatal(err)
+	}
+	tarw.Close()
+
+	// Check that the mtime is correct (#1808)
+	tarReader := tar.NewReader(buf)
+	hdr, err := tarReader.Next()
+	if err != nil {
+		t.Fatal(err)
+	}
+	testutil.CheckDeepEqual(t, mtime, hdr.ModTime)
+}
+
 func setUpFilesAndTars(testDir string) error {
 	regularFilesAndContents := map[string]string{
 		regularFiles[0]: "",