He dig out tiny caves, and store gold and silver in them. He also build bridges of silver and make crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs his help, yet many people are afraid to let him help them. Who is he ?

It is like a doll, But not for playing;
It stand up straight, but if windy it may be laying;
It is well-known and famous for having no brain;
It work outside both day and night in sunshine or in rain.
What is it ?