Implementing Multi-Stage Builds for Slim Images

Learners will construct multi-stage Dockerfiles to compile applications in one environment and copy only the final artifacts into a minimal production image.

In this lesson, you will learn how to shrink your production images by using multi-stage builds to separate your compilation tools from your final application binary.

The Problem: Bloated Images

Up until now, you’ve learned how Docker creates layers and stores them in an immutable filesystem. If you are building a language like Go or Java, your image needs a heavy "build environment"—complete with compilers, source code, and build caches—to turn your code into an executable. Including these tools in your final image is unnecessary for running the app, increases security risks, and wastes storage.

The Solution: Multi-Stage Builds

Multi-stage builds allow you to use multiple FROM instructions in a single Dockerfile. Each FROM instruction starts a new "stage."

The Build Stage: You use a heavy image containing everything needed to compile your code.
The Final Stage: You start a clean, minimal image (like alpine or scratch) and use the COPY --from command to reach back into the previous stage and grab only the finished executable.

Here is what that looks like in practice:

# Stage 1: The Build Environment
FROM golang:1.21 AS builder
WORKDIR /app
COPY . .
RUN go build -o my-app main.go

# Stage 2: The Production Environment
FROM alpine:latest
WORKDIR /root/
# Copy only the compiled binary from the 'builder' stage
COPY --from=builder /app/my-app .
CMD ["./my-app"]

Why This Works

When Docker finishes the build, it only keeps the layers from the final stage. The entire golang compiler, the source code files, and all the temporary build artifacts from the first stage are discarded. The resulting image contains nothing but your binary and the tiny operating system files required to run it. This keeps your production images lean, fast to deploy, and significantly smaller in size compared to single-stage builds.

Key Takeaway: Multi-stage builds act as a filter, allowing you to use heavy tools to compile your application while ensuring only the necessary production artifacts persist in your final, slim image.