I am looking for evergreen shrubs to grow in two half barrels that frame my garage door.
Shrubs for Growing in Half Barrel
Certified GKH Gardening Expert
Boxwoods are popular for this. They grow slowly so do not need to be repotted often. You also might try looking at Inkberry. It has a similar look to boxwood. It grows slowly as well, but is typically a little more cold hardy than boxwood and can take the winter better.
This article will help with the appropriate soil mix for your container evergreens: https://www.gardeningknowhow.com/special/containers/proper-soil-mix-for-evergreen-container-plants-and-trees.htm