message: Nydusify: add some comments for mount
  parseSourceMount parses mounts object returned by the Mount method in
  SourceProvider, checks diff layer type then decides to use which kind
  of whiteout spec applier in nydus-image during building. For example,
  `overlay` mount type for containerd in buildkit, `oci-directory` for
  unpacked OCI layer.
diff: @@ -54,18 +54,25 @@ type buildLayer struct {
  bootstrapPath string
  }
  +// parseSourceMount parses mounts object returned by the Mount method in
  +// SourceProvider, checks diff layer type then decides to use which kind
  +// of whiteout spec applier in nydus-image during building. For example,
  +// `overlay` mount type for containerd in buildkit, `oci-directory` for
  +// unpacked OCI layer.
  func parseSourceMount(mounts []mount.Mount) (*sourceMount, error) {
  if len(mounts) == 0 {
  return nil, errors.New("Invalid layer mounts")
  }
  switch mounts[0].Type {
  + // For containerd mounted layer
  case "bind":
  return &sourceMount{
  Source: mounts[0].Source,
  WhiteoutSpec: "overlayfs",
  }, nil
  + // For containerd mounted layer
  case "overlay":
  var prefix = "lowerdir="
  for _, option := range mounts[0].Options {
  @@ -84,6 +91,7 @@ func parseSourceMount(mounts []mount.Mount) (*sourceMount, error) {
  mounts[0].Options,
  )
  + // For unpacked OCI layer
  case "oci-directory":
  return &sourceMount{
  Source: mounts[0].Source,
  