Air comming in from door side
The general fix for air leaking around a door is known in the US as "weatherstripping".



Image from: http://www.energyconscious.com/m-d-building-products-door-weather-stripping-white-17-63628.html

It comes in several shapes and sizes, but the basic idea is to fill a gap with a compressible foam that will close an irregular gap and keep air out.  It's good practice to put it all the way around a door.