Look for food safe containers. A water butt used for storing rainwater won't be food safe, nor will that 55 gallon oil drum you picked up from the junkyard.
If a container is labelled as food safe, bad things happen to the company should it be proven that it is NOT food safe, so a good quality, food-safe container is your best option. Unless you want to look into rainwater harvesting, with particle filtration, UV sterilisation, reverse osmosis system for chemical removal, and a portable generator to power it all.