Your flowering dogwood might be unhappy in its pot, because these plants don't like drying out. I suspect that moisture is more of a problem than excess sunlight. Dogwoods require partial shade and naturally grow in locations with rich, well-drained, slightly moist soil. Dogwoods are natives of eastern woodlands and do best in a similar environment. They need protection from hot afternoon sun. Your dogwood should do better when you plant it in the ground. If your soil is alkaline, amend it with peat moss or other types of organic matter before planting your tree. Water it well after planting. Be sure the soil has no chance to dry out, especially for the first few years, but avoid overwatering.