I've grown spuds in bags for a few years now and really rate it if growing for one or two people. I bought bags from Willkinsons, cheap and very durable.
Builders bags may be too pourus, ie when you water, the sides don't retain water and it floods out. Small recycle containers which have holes in the bottom are good for spud growing, that's if you don't want the expense of buying custom spud bags.
I put crocks in the bottom of bags for drainage and a min layer of compost, spuds like seaweed, so if you can obtain some seaweed washed up on the beach put some in at this stage and put your chitted spuds on top with a good layer of MPC or home grown compost...half fill the spud bag with your choosen poison.
Label the bags, 1st, 2nd or main and as shoots start to push through the compost keep topping up the bag, making sure you don't break the spud shoots.
I put 5 to 6 spuds in each bag and get a varible crop depending on the variety. Grew 6 bags last year and didn't buy a spud from July through to December.