Fix ci

[dannystaple]

• Add a docker testing image - currently py 3.11, but will add other variants.
• Fix failures found when running make check, make shellcheck and make qa. These do not result in any functional changes.
• Fix test mocking issue in tox.ini. Tests and functionality unchanged.
• Add a CONTRIBUTING.md guide, showing how to run these checks within the docker env.
• Minor fix to checks - exporting the TERM settings.

This definitely overlaps with #14 , and I forgot about the chat with @Gadgetoid there. So similar changes coming for https://github.com/pimoroni/boilerplate-python.

A couple of whys

• Why dockerfile? Because on a desktop, I don't have GitHubs setup python, and this would be a reasonable way to get to the matrix of versions. Plus it's got the other bits it needs (presuming I haven't missed some, we'll find out).
• Why uv - managing a python setup with lockfiles means fewer package update surprises. It does need to be relocked sometimes though.
• Why a lockfile - so we get fewer moments of "ahhh... this is no longer compatible with todays build of smbus", at least until we relock because we meant it.
• Why a contributing doc - so people can run this stuff at desk without having to discover it all again.

Mostly Developer Experience (devx) to make fixing the ci failures a bit easier.

Testing

• Make check, make shellcheck, make qa and make pytest were ran locally on this - python 3.11 at this point.
• CI Workflows for the rest of the matrix are now green (The prior merge was not).