Build an application Docker image from the package

Assuming the package is runnable as an application ([docs](https://github.com/jenstroeger/python-package-template?tab=readme-ov-file#package-or-application)), adding a Makefile goal that builds the Docker image would be useful. Extra cookie points for using [Nix](https://nixos.org/) to build a minimal image ([discourse](https://discourse.nixos.org/t/building-a-python-docker-image-in-nix/51298/6)).