Use `uv` in backend container

[yashgorana]

Description

• Use uv as package manager
• Cleaned up dockerfile & dockerignores

Affected Dependencies

List any dependencies that are required for this change.

How has this been tested?

• Describe the tests that you ran to verify your changes.
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Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels
• [x] My changes are covered by tests

[yashgorana]

Haven't tested this rigorously so please don't merge

[rasswanth-s]

Great work , I tested the builds

on DEV branch without Cache: Took 211 s

on this branch without Cache: took 115 s

That's a 2x a speed up great work