Ability to cache Python downloads in Docker build

[merlinz01]

Question

Is there a way to cache what is downloaded for managed Python installations? As far as I can tell, uv either downloads and extracts in-memory to UV_PYTHON_INSTALL_DIR, or downloads to a temp file, extracts, and then deletes it. Neither would make it possible to cache the Python download in Docker with RUN --mount=type=cache. So with UV_CACHE_DIR cached, even though the package installation is very fast, it has to download Python every time the packages are installed, slowing down the build.

It doesn't work to cache-mount UV_PYTHON_INSTALL_DIR because then the Python binary isn't included in the final image.

This is the relevant part of the Dockerfile I'm using:

# syntax=docker/dockerfile:1-labs
FROM debian:trixie-slim
RUN apt-get update && \
    apt-get install -y ca-certificates git && \
    update-ca-certificates && \
    apt-get clean && \
    rm -rf /var/lib/apt/lists/*
COPY --from=ghcr.io/astral-sh/uv:0.7 /uv /usr/bin/uv
RUN useradd myuser -d /app -M -U -u 1000
USER myuser
WORKDIR /app
COPY pyproject.toml uv.lock ./
RUN mkdir -p .cache/uv
RUN --mount=type=cache,uid=1000,gid=1000,target=/app/.cache/uv \
    uv sync --compile-bytecode --frozen --no-dev --link-mode=copy

As you can see it takes 2.8 seconds downloading Python:

 > [stage-0  9/12] RUN --mount=type=cache,uid=1000,gid=1000,target=/app/.cache/uv     uv sync --compile-bytecode --frozen --no-dev --link-mode=copy:
0.390 Downloading cpython-3.13.5-linux-x86_64-gnu (download) (33.8MiB)
3.173  Downloading cpython-3.13.5-linux-x86_64-gnu (download)
3.305 Using CPython 3.13.5
3.305 Creating virtual environment at: .venv
3.529 Installed 102 packages in 208ms
4.443 Bytecode compiled 3072 files in 912ms
4.443  + aerich==0.9.2
4.443  + aiofiles==24.1.0
4.443  + aiohappyeyeballs==2.6.1
4.443  + aiohttp==3.12.3
4.443  + aiosignal==1.3.2
...

What I would like is if there was something like $UV_PYTHON_DOWNLOAD_CACHE_DIR (default to $UV_CACHE_DIR/python-download-archives) that is where all the Python distributions (cpython-3.13.5-linux-x86_64-gnu.tar.gz etc) are downloaded to and left intact, then used for subsequent Python installs.

Possibly related:
#13304
#7865
#8025
#14265
#6212
astral-sh/setup-uv#621

Platform

Linux 6.1.0-39-amd64 x86_64 GNU/Linux

Version

uv 0.7.22