The term decon container refers to the container from a M258A1 skin decontamination kit. The kit includes six decontamination packets and a plastic container. The empty containers are available from military surplus dealers.
Decon containers are one of the definitive containers for a small size cache. They are waterproof when closed properly, and are durable and relatively inexpensive.
The solutions in the decontamination packets are caustic, and warnings are molded into the plastic lid of the container, and are printed on an adhesive label on the container itself. Before using a decon container for a geocache, you should sand off the warnings molded into the lid, and peel off (or cover up) the adhesive warning label.