If there was no fire escape, I'd probably Crouch down in the dumpster, hoping that they can't figure out how to get to me, or that the dumpster is too tall for them to clamber into. If there is a fire escape overhead, as if it was an architecturally sound alley that is up to code, I would try and get up at least one level higher than the shamblers below me. Zombies are notorious in their hatred of both ladders and stairs, their most oppositional arch-nemesis, thwarting many of their decaying schemes.