A dumpster makes perfect sense Nobody would look in a dumpster. If it is nearly empty the Sides would work like a big drum to make the boom louder. They are easy to access.
Plus the object of terrorism is to scare people. So It works just fine to put a bomb in a dumpster
The side of the dumpster was crushed inward, not outward.
It appears the device was next to or under the dumpster.