Aluminum foil alone should not be a concern. Cooks use the product all the time for numerous purposes. The issue is in the temperature range of stored food (including potatoes) after cooking. When you store any food it’s best to allow the heat to fully dissipate prior to enclosing in (any) container. The food safety danger is when the core of saved food cannot cool down enough to a safe holding temperature. Bacteria can grow in this ‘danger zone’ between 40 and 140°F (4 and 60°C). So, either keep foods hot (above 140°F) or store chilled (below 40° F) and consume within two days. Unsure about the temperature or time stored? Err on the side of safety, do not consume suspected contaminated food and discard.