Boxes as in the typical tetrapaks. They're very much like the regular milk cartons I've seen sold in the US, and basically built the same way but have an additional very thin aluminum foil layer in between the cardboard and plastic, to provide a more complete barrier against light and oxygen, and nowadays they even sell them with screw caps and what not.

Either option still seems much better than plastic jugs, to me that just screams liquid laundry detergent or motor oil.