It's nearly impossible to permanently eliminate rats around a dumpster, you just have to keep the numbers down out there and prevent them from getting inside. We had an outside rat problem at one restaurant I worked at. Traps and poison worked to a degree, but still didn't to the trick. Finally, someone (and I refuse to stay who by way of the Fifth Amendment and out of fear of PETA) placed a pan with a mixture of flour and Nesquik out back by the door, and placed a pan of water right next to it. The next night the same thing happened. Again on the third night, same thing. Then, on the 4th night, the flour and Nesquik mixture was cut about 50/50 with Quikrete dry cement.
Next night, no more rats. Not for many nights.