I've mentioned my method of using a dry bag as a "portable washing machine" before. IMO definitely preferable to stomping on my clothes on the shower floor.
I use a 12 liter dry bag to wash my clothes in, rather than the albergue laundry sinks. As I'm undressing for my shower and the water is warming up I put half a laundry detergent sheet
and water in the bag, then my clothes. Then I fill the bag about 3/4 full with water and close it up. I give it a few shakes and set it aside to soak while I shower and dress. I then shake the bag some more to agitate everything well before rinsing in the laundry sink. The detergent sheets don't create a lot of suds, but get the clothes clean. After wring them out well I roll them up in my towel and twist it. This method gets my clothes cleaner than using the laundry sinks alone in the albergues. I think that the long soaking time is the secret. I even use the dry bag to wash when I'm staying in a room with my own bathroom.