Upgrade Docker Images from Debian Buster to Bullseye

[jbpaux]

Current Docker images build against Debian Buster release which is in debian terminology the "oldstable" version.
It was released in 07/2019 and supported until 09/2022 and is currently in LTS until 07/2024.

Bullseye, the current "stable" version was released 08/2021 and is planned in support until 08/2024.
The new Bookworm version is supposed to come in soon (summer time).

I think we should at least upgrade the container images to bullseye.

Let's see how it looks in the build process.
I'll do PR.