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?

Review the current devcontainer configuration and the referenced devcontainers/images/src/python image. Test startup time and available tools, including zsh, pipx, and the common-utils feature, then compare the listed pros and cons. Done means recording a clear decision and updating the devcontainer configuration if the Python image is selected.

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