Multi-stage Dockerfile

[Kludex]

• We want to have a different stages for development, staging (?) and production.
• We want to remove root rights from dockerfiles.

References:

• https://stackoverflow.com/questions/53835198/integrating-python-poetry-with-docker
• https://github.com/line/promgen/blob/master/Dockerfile
• https://github.com/python-poetry/poetry/discussions/1879#discussioncomment-216865
• https://pythonspeed.com/docker/