In a disused warehouse at an undisclosed location in Ukraine, a military drone instructor who goes by the call sign "DC" showed CBS News a makeshift practice course that students must learn to fly the increasingly indispensable devices through before they join the country's defense against Russia.  The trainees have to master the operation of the small but mighty First-Person-View (FPV) drones, which have become a signature weapon in the ongoing war sparked by Russia's February 2022 full-scale