Export pytest test coverage percentage into a shield for the README

**Is your feature request related to a problem? Please describe.**
I just want to see a shield of the test coverage for pgbelt. The tests run in a docker container, so getting the coverage info into default GHA actions is not as trivial.

**Describe the solution you'd like**
I want a shield on the README saying the pytest test coverage percentage.

**Describe alternatives you've considered**
Not doing this, it's still nice to have.

**Additional context**
I'd like a shield from shields.io.

Start by inspecting the Docker-based pytest workflow and the README to see where coverage is produced and where badges are documented. Determine how the percentage can be exposed to shields.io through the existing GitHub Actions setup; done means the README displays a shield with the current pytest coverage.

Written by the indexing model from the issue text.