[Build/CI] Revert back to Ubuntu 20.04, install python 3.12 with uv

[tlrmchlsmth]

Revert back to 20.04 to avoid breaking the wheel for users on OSes such as UBI 9 that have an earlier glibc version than 20.04.

Also use uv to install python 3.12 instead of deadsnakes.

[gemini-code-assist]

Code Review

This pull request aims to revert the build image to Ubuntu 20.04 for better glibc compatibility and to switch to uv for Python installation. The changes in the base stage of the Dockerfile correctly implement this, simplifying the Python setup.

However, there is a critical omission: the vllm-base stage (starting at line 271) has not been updated. It still uses the old deadsnakes PPA method for installing Python, which is inconsistent with the changes made in the base stage and the stated goal of the PR. To fully adopt uv for Python management and ensure consistency across the Docker build, the vllm-base stage must be refactored to use the same uv-based Python installation method.

Additionally, I've left one comment regarding an improvement for build script robustness.

[ProExpertProg]

Thanks for doing this!

[simon-mo]

UV not happy

[2025-10-02T18:21:16Z] #19 [base 10/10] RUN --mount=type=cache,target=/root/.cache/uv     uv pip install --system -r requirements/cuda.txt     --extra-index-url https://download.pytorch.org/whl/cu$(echo 12.8.1 \| cut -d. -f1,2 \| tr -d '.')
  | [2025-10-02T18:21:17Z] #19 1.133 Using Python 3.12.11 environment at: /root/.local/share/uv/python/cpython-3.12.11-linux-x86_64-gnu
  | [2025-10-02T18:21:17Z] #19 1.134 error: The interpreter at /root/.local/share/uv/python/cpython-3.12.11-linux-x86_64-gnu is externally managed, and indicates the following:
  | [2025-10-02T18:21:17Z] #19 1.134
  | [2025-10-02T18:21:17Z] #19 1.134   This Python installation is managed by uv and should not be modified.
  | [2025-10-02T18:21:17Z] #19 1.134
  | [2025-10-02T18:21:17Z] #19 1.134 Consider creating a virtual environment with `uv venv`.
  | [2025-10-02T18:21:17Z] #19 ERROR: process "/bin/sh -c uv pip install --system -r requirements/cuda.txt     --extra-index-url ${PYTORCH_CUDA_INDEX_BASE_URL}/cu$(echo $CUDA_VERSION \| cut -d. -f1,2 \| tr -d '.')" did not complete successfully: exit code: 2
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[simon-mo]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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[simon-mo]

I'll just merge this in given pip install is working

[2025-10-03T04:13:59Z] #19 [base 10/10] RUN --mount=type=cache,target=/root/.cache/uv     uv pip install --python /opt/venv/bin/python3 -r requirements/cuda.txt     --extra-index-url https://download.pytorch.org/whl/cu$(echo 12.8.1 \| cut -d. -f1,2 \| tr -d '.')
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