Which containers to boil dulce de leche for 7 hours? I want to make dulce de leche by boiling it. As the store bought cans leach plastics like BPA into the food, I want to avoid using it. 

What are jars/cans, preferably reusable that you can put inside some food, close it and boil for several hours and are made from safe for humans material?
This is exactly what Mason Jars are for.  They're glass, so they can stand long periods of heat without leaching, and are designed to be boiled.  Unlike a can, I'd suggest leaving the lid secured loosely so that it can bleed off pressure.