Please read the common steps first.
udocker is not Docker.
udocker provides four modes for running containers as a non-root user:
The third mode (Rn) falls into the scope of Rootless Containers, but the others are not: Pn and Fn are not containers (in our view), and Sn requires SETUID bit.
For futher information, see https://github.com/indigo-dc/udocker/