Fix Dockerfile python version and dependencies installation

[gabrielgaraujo]

Problem: The Docker configuration for running norminette is not working out of the box.

• Following PR #380, the minimum required version of Python is 3.8, but the Docker container was using Python 3.7.
• Dependencies are now managed by Poetry instead of a plain requirements.txt file (which has been removed, yet still referenced in the Dockerfile).
• Running the command stated in the README creates a new container each time, leaving unused containers behind.

Solution:

• Updated the Python version in the Dockerfile to the latest stable version (3.12).
• Updated the Dockerfile to use Poetry for dependency management.
• Added the --rm flag to the Docker run command in the README to automatically remove the container after execution.