fix: Specify Debian buster tag in base Docker images (!47)

This week (2021-08-16) debian:bullseye was released and all the Python base Docker images updated to it. However, not all packages that exist on debian:buster have been packaged and released for bullseye yet making bullseye unstable for builds. Example: libqt4-dev is currently not packaged for bullseye

$ docker pull python:3.8-slim
$ docker run --rm -ti python:3.8-slim /bin/bash
root@8f381a5cc9c3:/# date
Tue Aug 17 16:51:38 UTC 2021
root@8f381a5cc9c3:/# cat /etc/os-release 
PRETTY_NAME="Debian GNU/Linux 11 (bullseye)"
NAME="Debian GNU/Linux"
VERSION_ID="11"
VERSION="11 (bullseye)"
VERSION_CODENAME=bullseye
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
root@8f381a5cc9c3:/# apt-get update -y && apt-get install libqt4-dev
Get:1 http://security.debian.org/debian-security bullseye-security InRelease [44.1 kB]
Get:2 http://deb.debian.org/debian bullseye InRelease [113 kB]
Get:3 http://security.debian.org/debian-security bullseye-security/main amd64 Packages [25.4 kB]
Get:4 http://deb.debian.org/debian bullseye-updates InRelease [36.8 kB]
Get:5 http://deb.debian.org/debian bullseye/main amd64 Packages [8178 kB]
Fetched 8397 kB in 1s (6113 kB/s)   
Reading package lists... Done
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
E: Unable to locate package libqt4-dev
root@8f381a5cc9c3:/# 

To fix this for the time being specify that debian:buster images should be used by using the python:3.8-slim-buster tag.

This problem was brought to my attention when I was fixing another issue raised by @gstark in https://gitlab.cern.ch/scipp/collinear-w/collinearw/-/merge_requests/65.

* Specify base image OS of Python base images for greater long term stability
   - Use python:3.8-slim-buster tag until there is a `libqt4-dev` or later release on python:3.8-slim-bullseye