When a building is bombed, a group of volunteers known as the White Helmets rush to the scene to dig through rubble to find survivors. In a conflict known for its never-ending descent into depravity, this one group stands apart as true servants of humanity.    On the line to discuss their work is Orlando von Einsiedel, who directed the new Netflix documentary "The White Helmets." The film follows members of the Aleppo contingent of the Syrian Civil Defense Corps as they go on rescue and training missions.   The White Helmets are unarmed and apolitical. But as Russia and Syrian forces have intensified the battle for eastern Aleppo, the White Helmets have increasingly been a target themselves. In the last week alone, four of their bases in Aleppo have been targeted and they are often the victims of a bombing strategy known as "double tap" in which a second bomb is unleashed on a civilian target just as rescue workers are arriving on the scene.    In this conversation, director Orlando von Einsiedel -- whose credits include the documentaries Virunga and Skateistan -- describes the work of the White Helmets and his decision to make them the subject of his newest film.