message: Nydusify: dump manifest.json of source image to workdir
  We can find source image info by manifest.json, make debug easier.
diff: @@ -11,6 +11,7 @@ import (
  "encoding/json"
  "fmt"
  "io"
  + "io/ioutil"
  "os"
  "path/filepath"
  "runtime"
  @@ -296,6 +297,17 @@ func (remote *Remote) Pull() error {
  // Unpack layer tar file to directory and not ignore OCI whiteout file
  func (remote *Remote) Unpack(workDir string, callback func(string) error) error {
  + // Write manifest.json to source directory
  + manifestFile := filepath.Join(workDir, remote.source, "manifest.json")
  + manifest, err := json.Marshal(remote.sourceManifest)
  + if err != nil {
  + return err
  + }
  +
  + if err := ioutil.WriteFile(manifestFile, manifest, 0644); err != nil {
  + return err
  + }
  +
  sourceDir := filepath.Join(workDir, remote.source)
  cs := remote.sourceImage.ContentStore()
  