Docker cleanup

[jennydaman]

• use python:3.9.1-buster official base image
• remove unused and impeding files
  • docker-entrypoint.py: not useful, caused bugs
  • pfurl/testts/*: nothing useful
• adapt semvar, bumped version to 2.3.1 from 2.3.0.1
• automatic builds on Github Actions to cross-compile docker images targeting amd64, arm64, and ppc64le
  • master tagged as latest
  • tags are built and named after themselves
  • action needed: add docker bot account password to secrets in setttings
• freeze pip dependencies