Update base images to python:3.8-slim-bullseye

As brought up in MR !47 (merged) it was noticed that it is important to specify the base image OS in the tag. For reasons deteailed in the MR, at the time of the MR not all necessary packages has been packaged and distributed for debian:bullseye so buster was chosen for the time being. Switching over to python:3.8-slim-bullseye or python:3.9-slim-bullseye is desirable, but it needs to have all the build dependencies packaged for it. Once that has happened this project should switch.