[docs/integration/docker] add sha pinning tip (#10955)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
As requested in https://github.com/astral-sh/uv/issues/6565, this adds a
tip discussing the ability to pin the image to a specific SHA digest and
why it may be useful.
## Test Plan
<!-- How was it tested? -->
Start serving the documentation locally
```shell
uvx --with-requirements docs/requirements.txt -- mkdocs serve -f mkdocs.public.yml
```
Then navigate to http://127.0.0.1:8000/uv/guides/integration/docker/ to
see the tool tip being rendered properly
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Co-authored-by: Zanie Blue <contact@zanie.dev>
