[llvm] [GitHub][docker] Add python3 venv package to CI container (PR #161024)

[Baranov Victor via llvm-commits]

vbvictor wrote:

> LGTM.
> 
> I think we want to eventually move to separate containers for the clang-format/premerge/clang-tidy workflows to trim down on space (and preinstall necessary python dependencies), but this seems fine for now, especially since the premerge container already has clang-format/clang-tidy.

It actually doesn't have clang-tidy installed. I tried building with clanf-tidy locally and got 1.1GB -> 1.2GB image size. So as the first Iteration I think the 100mb increase is ok


https://github.com/llvm/llvm-project/pull/161024