Add platform for docker

[VladOS95-cyber]

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What does this PR do?

It is possible to specify platform param in the cmd args llama stack build --template local --image-type docker --platform linux/amd64 --name llama-stack

https://github.com/meta-llama/llama-stack/issues/253

Test Plan

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• tests you ran to verify your changes with result summaries.
• provide instructions so it can be reproduced.

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Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Ran pre-commit to handle lint / formatting issues.
• [x] Read the contributor guideline, Pull Request section?
• [ ] Updated relevant documentation.
• [ ] Wrote necessary unit or integration tests.