Armed carjackers in south St. Louis city forced a man to climb into a dumpster, and then took off in his vehicle, authorities say.The 25-year-old victim told investigators he was getting something out of the trunk of his car at about 1:10 a.m. on February 13 when the thieves sneaked up on him from behind. One of the crooks pointed a gun at him, while the other rifled the man's pockets and told him to hand over his car keys.…