Cleanup requirements, docker file

[sumpfork]

• get rid of requirements.in in favour of directly compiling pyproject.toml into requirements.txt
• add dev-requirements.txt in the same way, based on the dev requirements in pyproject.toml
• require a reasonable python version > 3.7
• simplify the docker file to
  • not pip-compile anything (it shouldn't, those versions are pinned for a reason)
  • not try to handle fonts (we can't distribute them)
  • not run --help by default - behaviour should be the same as the normal script
  • update the README to show how to map a the local dir so you can actually get the output file and use local fonts