[ci] Cleanup & refactor Dockerfile to pass different Python versions and sccache bucket via build args

[khluu]

• Clean up unused commands
• Pack dependencies installation into as few commands as possible so Docker can cache it better
• Install flashinfer based on Python version passed in instead of fixing it to 3.10
• Make SCCACHE_BUCKET and SCCACHE_REGION build arg (with default set to vllm's bucket and region)

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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Once the PR is approved and ready to go, please make sure to run full CI as it is required to merge (or just use auto-merge).

To run full CI, you can do one of these:

• Comment /ready on the PR
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🚀

[njhill]

@khluu maybe you could also check #7525 in case you didn't see it

[khluu]

@khluu maybe you could also check #7525 in case you didn't see it

My PR doesn't really touch the dependencies like #7525. It's more like a small refactoring to cleanup stuff & allow different sccache bucket or python version to be passed in via build args so we can run CI with different versions (for example flashinfer python version is fixed to 3.10 so we can't test anything else).

[simon-mo]

I think we lost TORCH_CUDA_ARCH_LIST but it is still needed to build cross platform wheels I believe

[khluu]

I think we lost TORCH_CUDA_ARCH_LIST but it is still needed to build cross platform wheels I believe

Ok I'll just bring it back.. we can clean it up later on