I usually bring a tiny eye drop bottle of unscented Dawn liquid detergent. One or two drops is all it takes for just about anything.
Other options included Dr. Bronners unscented liquid or bar or Clearly Natural unscented bar. These are vegetable based and the Clearly Natural is glycerin based. I use both at home. The Dawn gives the most bang for the buck, and seems to be the least toxic to critters. So little is needed to do the job.