This is a bug in Docker software and has nothing to do with DietPi. Using the forum search should have brought this up. 
DietPi-Software | Docker: Service fails to start after upgrade (AppArmor) · Issue #6126 · MichaIng/DietPi · GitHub
This is a bug in Docker software and has nothing to do with DietPi. Using the forum search should have brought this up. DietPi-Software | Docker: Service fails to start after upgrade (AppArmor) · Issue #6126 · MichaIng/DietPi · GitHub