chore(docker): upgrade base image to python3.12-slim

[melodysium]

Description of proposed feature

Currently, the manim docker image uses python3.8-slim. With the recent update to python3.12 in the main library, we should update the Dockerfile's base image to python3.12-slim to take advantage of the new support.

There's lots of nice features in python 3.9, 3.10, 3.11, and 3.12:

• lots of type hinting features (unions for dicts, collection generics, Self type, and Type Params and type keyword)
• pattern matching
• exception groups
• less restrictions on f-string contents

How can the new feature be used?

People who use the Docker distribution of manim (such as myself) can benefit from these new Python language features. In particular, one can make better use of Manim's recent type hint efforts.

Additional comments

I don't believe this should break anything; should be a simple one line change.