Add platform for docker

[VladOS95-cyber]

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

#253

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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[VladOS95-cyber]

Hi @ashwinb @yanxi0830 @hardikjshah @dltn @raghotham! This PR is ready for a review. Please, take a look.