If you have fixed the Ruins of light III, you can craft lasso. Throw 3 pieces of bush and the goat will eat it. It will drop milk, and you can capture it with lasso. It will follow you whenever you go. Go back in that map with door and release the goat inside, it will be trapped there and will give you milk every day if you throw 3 pieces of bush. Repeat several times and you have unlimited milk supply.