For a little while, a couple of years maybe, it'd be a stretch to 5 years if its a smallish plant initially, but it will inevitably need planting out after that.

Let's go one further than Bamboo, although what Bamboo says is correct. A Kilmarnock willow can grow into a large tree. Why not do as I did and buy a VERY large pot. I'm thinking about 24" across the top x about 36" high, or as large as you can comfortably get into the car and get home and lift yourself or with help.
Now borrow or get a friend to use a 5" whizzer and cut the bottom right off. Get the pot into position and bury it, only about 3" or so and set it on bricks and make sure the top is flat and level. Now break up the soil at the bottom and pot up your Kilmarnock as normal. It will possibly look a little silly for a couple of years but the roots will grow down through the bottom and into the soil and allow the tree to become big and established. The advantage is that the roots growing through the bottom will anchor the tree and prevent it toppling over in the high winds. The disadvantage is that if you move you will not be able to take it with you. This system works. I know of an example in Cyprus where an olive tree was planted about 1950 using the same method and is now an incredible sight whereby the tree has split the pot down the length and looks beautiful. Southportsmith

Most of the salix (willow family) prefer a moist soil, so if you intend growing in a pot, you will have to water on a regular basis during the growing season, but as advised, you will have to plant out eventually.