Use func from stdlib to clone *tls.Config for calming vet

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

server/server.go

@@ -113,6 +113,34 @@ func (s *Server) Serve() error {
 	return server.ListenAndServe()
 }
 
+// copy from net/http/transport.go
+func cloneTLSConfig(cfg *tls.Config) *tls.Config {
+	if cfg == nil {
+		return &tls.Config{}
+	}
+	return &tls.Config{
+		Rand:                     cfg.Rand,
+		Time:                     cfg.Time,
+		Certificates:             cfg.Certificates,
+		NameToCertificate:        cfg.NameToCertificate,
+		GetCertificate:           cfg.GetCertificate,
+		RootCAs:                  cfg.RootCAs,
+		NextProtos:               cfg.NextProtos,
+		ServerName:               cfg.ServerName,
+		ClientAuth:               cfg.ClientAuth,
+		ClientCAs:                cfg.ClientCAs,
+		InsecureSkipVerify:       cfg.InsecureSkipVerify,
+		CipherSuites:             cfg.CipherSuites,
+		PreferServerCipherSuites: cfg.PreferServerCipherSuites,
+		SessionTicketsDisabled:   cfg.SessionTicketsDisabled,
+		SessionTicketKey:         cfg.SessionTicketKey,
+		ClientSessionCache:       cfg.ClientSessionCache,
+		MinVersion:               cfg.MinVersion,
+		MaxVersion:               cfg.MaxVersion,
+		CurvePreferences:         cfg.CurvePreferences,
+	}
+}
+
 // ListenAndServeTLSWithSNI serves TLS with Server Name Indication (SNI) support, which allows
 // multiple sites (different hostnames) to be served from the same address. This method is
 // adapted directly from the std lib's net/http ListenAndServeTLS function, which was
@@ -123,10 +151,7 @@ func ListenAndServeTLSWithSNI(srv *http.Server, tlsConfigs []TLSConfig) error {
 		addr = ":https"
 	}
 
-	config := new(tls.Config)
-	if srv.TLSConfig != nil {
-		*config = *srv.TLSConfig
-	}
+	config := cloneTLSConfig(srv.TLSConfig)
 	if config.NextProtos == nil {
 		config.NextProtos = []string{"http/1.1"}
 	}