Update parent image to debian:bullseye-slim #3
Description
With the release of Debian 11 (bullseye) in August 2021, it would be nice to update our current parent image debian:sid-20200720-slim to debian:bullseye-slim.
One of the major changes in Debian 11 is the transition from Python 3.7 (or Python 3.8.5 in sid in July 2020) to Python 3.9, which implies the need to upgrade various Python libraries and to revise our existing Python scripts where necessary.
Would be nice to have to have GitHub Actions CI/CD set up first, though not strictly necessary in that order.