Change http.Header copy to builtin Clone

Signed-off-by: Jin Dong <djdongjin95@gmail.com>

remotes/docker/resolver.go

@@ -152,7 +152,6 @@ func NewResolver(options ResolverOptions) remotes.Resolver {
 		// make a copy of the headers to avoid race due to concurrent map write
 		options.Headers = options.Headers.Clone()
 	}
-
 	if _, ok := options.Headers["User-Agent"]; !ok {
 		options.Headers.Set("User-Agent", "containerd/"+version.Version)
 	}
@@ -547,9 +546,10 @@ func (r *request) do(ctx context.Context) (*http.Response, error) {
 	if err != nil {
 		return nil, err
 	}
-	req.Header = http.Header{} // headers need to be copied to avoid concurrent map access
-	for k, v := range r.header {
-		req.Header[k] = v
+	if r.header == nil {
+		req.Header = http.Header{}
+	} else {
+		req.Header = r.header.Clone() // headers need to be copied to avoid concurrent map access
 	}
 	if r.body != nil {
 		body, err := r.body()