milk bottles are perfectly ok IF AND ONLY IF ALL THE SUGAR IS GONE, OR ALL THE YEAST IS DEAD!!
If you've racked off enough times, and fermentation has stopped due to lack of any available sugar at all, and you've added camden tablets, and potasium sorbate, (or pasteurised) then the cider will be inert, and will be fine in milk bottles. They have the advantage of being squishable, so as you drink a glass, you can exclude most of the air, and close the lid.
If there is any chance at all of there being any sugar left, or any yeast still alive - DO NOT USE PLASTIC MILK BOTTLES.