Decide whether to base the devcontainer on `mcr.microsoft.com/devcontainers/python`

See https://github.com/devcontainers/images/tree/main/src/python

Pros:
- Already has `zsh` so should be a lot quicker to start a new devcontainer (with `common-utils` feature) - **needs testing**
- Contains additional tools like `pipx`

Cons:
- More change
- Another tie into Microsoft ecosystem

@callumforrester @garryod thoughts?