Certified GKH Gardening Expert
Spring or fall is the best time for planting lilac bushes. They grow just fine in containers, though you want to make the container about 5"-10" wider than the rootball of the plant you have and at least 2' deep.
Dwarf types are also better choices, such as dwarf Koren lilac. They spread nicely, but stay small so it would not outgrow the container too quickly.
Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-care-growing-and-planting-lilac-bush-plants.htm