fix: outdated and broken python minor version in Dockerfile

[matfax]

The current Docker upstream image it outdated and causes build issues on rebuilds:

   > [5/7] RUN apt-get update && apt-get install --no-install-recommends -y curl git && apt-get clean && rm -rf /var/lib/apt/lists/*:
  0.165 Err:4 http://security.debian.org/debian-security buster/updates Release
  0.165   404  Not Found [IP: 146.75.30.132 80]
  0.167 Err:5 http://deb.debian.org/debian buster Release
  0.167   404  Not Found [IP: 146.75.30.132 80]
  0.174 Err:6 http://deb.debian.org/debian buster-updates Release
  0.174   404  Not Found [IP: 146.75.30.132 80]
  0.178 Reading package lists...
  0.185 E: The repository 'http://security.debian.org/debian-security buster/updates Release' does not have a Release file.
  0.185 E: The repository 'http://deb.debian.org/debian buster Release' does not have a Release file.
  0.185 E: The repository 'http://deb.debian.org/debian buster-updates Release' does not have a Release file.

Moreover, the ENV instruction without = is deprecated.

LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 3)
LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 4)
LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 2)

This PR solves the rebuild issues and bumps Python to the newer and still maintained version 3.11.