Why Named build stages in Docker? - Purpose & Use Cases

The Big Idea

Discover how naming your build steps can save hours and gigabytes in your Docker projects!

The Scenario

Imagine you are building a complex Docker image by running many commands one after another in a single Dockerfile. You want to create a smaller final image by removing unnecessary files and tools used only during building.

The Problem

Doing all steps in one big build makes the image large and slow to build. If you want to change one step, you must rebuild everything again. It is hard to organize and reuse parts of the build, leading to wasted time and disk space.

The Solution

Named build stages let you split the Docker build into clear parts with names. You can build once, then copy only what you need from earlier stages to the final image. This makes builds faster, images smaller, and your Dockerfile easier to read and maintain.

Before vs After

✗ Before

FROM ubuntu
RUN apt-get update && apt-get install -y build-essential
RUN make all
RUN rm -rf /var/lib/apt/lists/*

✓ After

FROM ubuntu AS builder
RUN apt-get update && apt-get install -y build-essential
RUN make all
RUN rm -rf /var/lib/apt/lists/*
FROM ubuntu
COPY --from=builder /app/bin /app/bin

What It Enables

Named build stages enable fast, efficient, and clean Docker images by reusing build steps and separating build from runtime.

Real Life Example

A developer builds a multi-stage Dockerfile where the first stage compiles code with all tools, and the final stage copies only the compiled app, resulting in a tiny image perfect for production.

Key Takeaways

Manual single-stage builds create large, slow images.

Named build stages split the build into reusable parts.

This leads to faster builds and smaller final images.