The problem

using the action, github first builds the Dockerfile, which references python:3.10 without any mention which version of debian to use. Debian 12 bookworm has been released recently, and a python:3.10-bookworm published, which is now used.
But the Dockerfile then tries to install git from bullseye-backports, which would downgrade the git package, resulting in the following errors, probably for every single user out there (unless there's some caching going on):

E: Packages were downgraded and -y was used without --allow-downgrades.
ERROR: failed to solve: process "/bin/sh -c echo "deb http://deb.debian.org/debian bullseye-backports main" >> /etc/apt/sources.list; apt-get update; apt-get install -y git/bullseye-backports" did not complete successfully: exit code: 100

Expected behavior

action should work

Additional context

proposed fix for now is to specify the version of debian to use by pinning to python:3.10-bullseye as base image.
Long-term you'll want to adopt python:3.11-bookworm, but that's a completely different matter.