It seemed that the dumpster is best positioned on a slope so that the hatch is facing downhill. That way every nudge on the thing also moves it uphill giving it upward momentum and prevents it from flipping over as you're hopping away. Ghosts are a good way to move dumpsters around. Use the boost if you need to flip it over, and carefully nudge it around to move or turn them.
Nah, that's campaign. A weird glitch in the physics engine allows two players jumping inside a dumpster like crazy for the dumpster to actually take flight. I guess it has to do with how the charaters don't exert any extra force downwards as they jump, so two of them banging their heads against a dumpster's insides will give it more force upwards than falling down, and when done correctly it will overcome the weight of the dumpster itself.