Speed up builds with BuildKit cache mounts

Approach

Mount the package manager's cache directory with --mount=type=cache. It persists between builds on the same builder but never ends up in the image.

Solution

# syntax=docker/dockerfile:1
FROM python:3.14-slim
WORKDIR /app
COPY requirements.txt .
RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt
COPY . .
CMD ["python", "main.py"]

Pitfalls

• Adding --no-cache-dir together with a cache mount (it disables the benefit).
• Expecting cache mounts to survive on ephemeral CI runners; export caches with --cache-to.

Variations

• npm: --mount=type=cache,target=/root/.npm; Go: /go/pkg/mod and /root/.cache/go-build; Maven: /root/.m2.
• In GitHub Actions, combine with cache-from: type=gha and cache-to: type=gha,mode=max.

Further reading: Optimize cache usage in builds