Replace pip with uv for faster dependency installation in Linux images (#162)

Use uv as the package installer in Linux (full + slim) Dockerfiles and
entrypoint. The PYPI_URL/PYPI_INDEX_URL env vars are preserved for
backward compatibility and translated to UV_INDEX_URL/UV_INSECURE_HOST.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>