Dumpsters that small surely must be front-load, as opposed to roll-off. As far as I know, neither have drain plugs for water to run out. With the roll-off, you have to open the end door, which on the bigger units, depending on the size and condition of the unit, can be difficult for one person -- especially if it is not placed on a flat surface.
The two biggest maintenance issues on dumpsters are welding holes and painting. Will the cleaning be performed prior to welding? If not, perhaps the runoff would go through the unrepaired holes.
Also, when cleaning the long, narrow roll-offs, it is easy to walk into the unit via the open end hatch; with the front-load, you may get a face full of water if you must clean them from outside. Hopefully, you are not a short guy, too, because they typically are 5-6' high.
I would imagine that size of front-load dumpster is primarily for food service, so buy lots of degreaser.