Ordinary fertile garden soil with added garden compost or well rotted cow manure will be fine. Also add a balanced granular fertiliser or, preferably one with a slightly higher phosphate content, say a ratio of 1-2-1 NPK.
If you have to use heavy loam add some grit sand to aid swift drainage.
Fill your container to about 40cm from the top, stir in the fertiliser (a good handful per container will suffice, place your tubers and then cover with around 12cm of the soil mix, keeping some back to add a couple of times as the stems grow, eventually leaving around 7cm at the rim for watering.
If you are planting first earlies and the containers are exposed have some hessian or old carpeting handy to cover in the likely event of hard frost.