I hate to tell you this - but melamine IS a plastic. And it won't hold up to exposure to the sun. There may be UV-stabilized melamine out there - but I've never run across any that I know of.

What are you planning to grow and how large a container are you planning to grow in? I've never seen a kitchen container of any sort available at a supermarket that was large enough to grow plants in. Maybe African violets or bromeliads - but not anything much bigger than that.

Are you sure you meant melamine? I don't think I've ever seen food containers made of melamine, just plates and cups and the like. Are you in the USA or somewhere else?

Also if you're growing food crops, they need to be outside all day at least, not just set out when its sunny or its raining.

Moving them in and out frequently causes more stress than just leaving them out - unless you're going to get a nightly frost or something.