Fix Dockerfile: Update Ubuntu version and correct ownership

[puneetred]

This pull request addresses two issues in the Dockerfile located in pwn_docker_example/challenge/Dockerfile:

1. Updated Ubuntu Version: The base image was changed from ubuntu:19.10 (which is EOL) to ubuntu:22.04, ensuring compatibility and support.
2. Corrected File Ownership: The ownership of /home/ctf is updated to the correct user ctf to resolve permission issues when running the container.

These changes improve the Dockerfile's functionality and ensure the container works as intended.