Multi-stage builds — much smaller Docker images

A Python application image weighing 1.2 GB is not normal — it’s a lack of multi-stage build.

The problem

When you install compilers, build tools, and development dependencies in the same image that goes to production, all of it travels together.

The solution

# stage 1: build
FROM python:3.12-slim AS builder

WORKDIR /app

RUN apt-get update && apt-get install -y --no-install-recommends \
    build-essential \
    && rm -rf /var/lib/apt/lists/*

COPY requirements.txt .
RUN pip install --user --no-cache-dir -r requirements.txt

# stage 2: runtime
FROM python:3.12-slim
WORKDIR /app
COPY --from=builder /root/.local /root/.local
COPY . .
ENV PATH=/root/.local/bin:$PATH
CMD ["uvicorn", "main:app", "--host", "0.0.0.0"]

The COPY --from=builder is the key: only what you explicitly copy leaves the first stage.

Measuring the result

docker build -t app:antes -f Dockerfile.old .
docker build -t app:depois .
docker images | grep app

In a real FastAPI project, this brought the image down from 1.1 GB to less than 180 MB — faster deployment, smaller attack surface, lower registry costs.