Update worker Dockerfile image to bookworm (#460)

* Update `worker` `Dockerfile` image to `bookworm`

The issue here was that `slim-buster` has reached LTS and so
trying to run `apt-get` commands on the image was returning 404
errors. The real robust solution is to upgrade to a newer future-
proof image like `bookworm` instead.

* Update to `python-3.12`

* Use Python 3.10

1. We’re capped at 3.10 due to the `custom_setup` tests which use Celery 4.
2. The examples CI is bugged as it does not use the current branch’s changes and should work once the PR is merged to `main` (if everything else passes)

---------

Co-authored-by: Tomer Nosrati <[email protected]>

Author: LeoTheMighty

src/pytest_celery/vendors/worker/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.10-slim-buster
+FROM python:3.10-slim-bookworm
 
 # Create a user to run the worker
 RUN adduser --disabled-password --gecos "" test_user