Update Python in Docker image

[wardwygaerts]

I saw that the Python version in the Docker image is still 3.6.12.

Is it a possibility to change FROM python:3.6-slim to python:3.9-slim (https://hub.docker.com/_/python) ?

[bugy]

Hi @wardwygaerts good idea! I'll do it soon

[bugy]

Done, dev version is based on 3.9 now

[wardwygaerts]

thanks, working fine!