Tooling and CI for code quality

[mk-mxp]

With #130 we now have a "production" build process for the Docker image. And can add a "development" build process to CI, including all the tooling we love in PHP world:

• code styling (PHP CodeSniffer or php-cs? PSR-12 or PER?)
• PHPStan, Psalm and/or Phan
• additional unit testing, mutation testing
• etc.

[mk-mxp]

Make use of assert during CI runs. See #130 (comment) for reasoning.