php-worker support memcached extension

.env.example

@@ -288,6 +288,7 @@ PHP_WORKER_INSTALL_GNUPG=false
 PHP_WORKER_INSTALL_PGSQL=false
 PHP_WORKER_INSTALL_MONGO=false
 PHP_WORKER_INSTALL_BCMATH=false
+PHP_WORKER_INSTALL_MEMCACHED=false
 # PHP_WORKER_INSTALL_OCI8 Does not work in php5.6 version
 PHP_WORKER_INSTALL_OCI8=false
 PHP_WORKER_INSTALL_PHALCON=false