If you store your water barrel in a place where you know you'll eventually need to move it, you may want to invest in a dolly at a local hardware store. One gallon of water weighs 8 pounds, therefore a full 55 gallon water barrel weighs 440 pounds. Plastic absorbs whatever is stored in it so it becomes impossible to wash all the milk and juice out. The absorbed milk and juice can leak back into the water causing it to become sour. Harmful bacteria can grow in such an environment.
Milk jugs are also biodegradable and designed to break down over a short period of time.
Plastic 2 liter pop bottles and glass juice bottles are okay to use as long as you clean them thoroughly before storing water in them. Plastic may absorb the pop, but it doesn't sour. It will only make your water taste a little sweeter. Glass containers will break in some emergencies so carefully consider where to store it. Rotate your water storage at least once a year.