[Python] --mount=type=cache doesn't speed up pip install during Docker build

I'm using --mount=type=cache in my Dockerfile to cache pip packages between builds, but it doesn't seem to have any effect on build speed. I expected pip to reuse cached packages and avoid re-downloading them, but every build takes the same amount of time.

Here is the relevant part of my Dockerfile:

FROM python:3.13-slim AS compile-image

RUN python -m venv /opt/venv
ENV PATH="/opt/venv/bin:$PATH"

RUN pip install --no-cache-dir --upgrade pip


# Final image
FROM python:3.13-slim
ENV PATH="/opt/venv/bin:$PATH"

COPY --from=compile-image /opt/venv /opt/venv

WORKDIR /app
ENV HOME=/app

COPY . .

RUN --mount=type=cache,mode=0755,target=/root/.cache/pip \
pip install -e .


Shouldn't this cache downloaded packages and reuse them in subsequent builds? What could be the reason it's not working as expected?

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