ci: fix slow test; pin docker-library/python repo#747

mikemccracken merged 1 commit intoproject-stacker:mainproject-stacker/stacker:mainfrom

mikemccracken:2025.12.12/main/fix-slow-test-convertmikemccracken/stacker:2025.12.12/main/fix-slow-test-convertCopy head branch name to clipboard