Asked about A Dockerfile that builds fast and runs safely
Why should a container not run as root?
An answer that works
If the application is compromised, root inside the container makes it easier to damage mounted data or exploit a container escape. A normal user limits what an attacker gets.
Reading an answer is not being able to give one. The section that teaches this is A Dockerfile that builds fast and runs safely, 4 hours of Containers.
Learn this properly