Yes, mints do spread once they are established and can be a real pest in the garden.* Mints have square stems so beware of plants with square stems as they are in the mint family.* I planted my mint in a half whiskey barrel and it survives my zone 7 winters here in Maryland.* You can use a mixture of organic potting soil and compost and your mint will love it.* You could also plant other herbs in there too.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.