
The container is higher up so you may need some boxes to make it easy for you to go up there: Stay in cover, like on the screenshot, and when he comes closer spam the Takedown button (by default it should be Q) until you knock him down. If you want to continue to be unnoticed, one idea is to hide as I do on this screenshot, show just for a second or two until this guard becomes suspicious (not alarmed!) and comes to investigate. There is one particular guard that may give you a hard time if you want to play stealthy. This is where the container is located in Loading Bay 2:


Since you are on the second story container the game detects as if you are on the second floor of Loading Bay 2. This is the exact location of the barrel:
