Isolate tests for Dockerfile builds using Poetry build includes/excludes

[d33bs]

Here's another possible change at your discretion. This would make it a bit easier with the context management.

1. Move ./build/docker/Dockerfile to ./Dockerfile
2. Add the tests directory as an include for the sdist build artifact but not wheel (instructions)

I think the end result would be that the commands docker build . --target testing would work both in a cloned repo root or in an extracted sdist.

Originally posted by @kenibrewer in #395 (comment)