Refactor the Dockerfile to alias pinned images

[rbtr]

Description

Refactor the Dockerfile to pull all external image refs to the top and pin them to SHA refs, then use them by alias throughout the build stages.

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• [ ] I have read the contributing documentation.
• [ ] I signed and signed-off the commits (git commit -S -s ...). See this documentation on signing commits.
• [ ] I have correctly attributed the author(s) of the code.
• [ ] I have tested the changes locally.
• [ ] I have followed the project's style guidelines.
• [ ] I have updated the documentation, if necessary.
• [ ] I have added tests, if applicable.

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