Upgrade Python version and packages

Improve Dockerfile by pinning OS packages version and makeing it more generic. Implement a build argument for tests (which currently don't exist).

Upgrade Poetry (its requirements file) to v1.0.0b3 and Gunicorn to v20 (including necessary OS packages in Dockerfile).

Improve pipeline by installing the many flake8 plugins using poetry and also going into backend subdir first (to make use of specific dot files)