Saddle bag for food container
It sounds like you need something like a handlebar bag or a rack trunk like these:

rack trunk

handlebar bag

The rack trunk would require adding either a seatpost mounted rack or a full rear rack. Either one would allow you to move to panniers in the future if you needed to carry more goods like a laptop or a change of clothes, but a full rack would be able to handle more weight and is generally recommended for stability and versatility.

The handlebar bag may or may not require a front rack, depending on how big it is. Some come with enough of a mount that you don't need anything additional to support the weight.