HDPE is not automatically food-safe. It depends on what additives the manufacturer used to colour it and stuff. However, it's not really an issue here because it will not be in contact with the grain.
I use a cheapo black plastic dustbin and it's fine. Keep the grain in the original sack and put that inside the bin. The bin will give a bit of extra protection from humidity, rodents, small children and accidental damage from, say, the lawnmower when I am putting it away next to the grain store....
