What is Multi-Stage Builds in Docker?

Multi-stage builds in Docker allow you to create smaller and more efficient images by using multiple FROM statements in a single Dockerfile. This feature is particularly useful for applications that require a build process, as it enables you to separate the build environment from the final runtime environment. By doing so, you can include only the necessary artifacts in the final image, significantly reducing its size.

1. Why Use Multi-Stage Builds?

Multi-stage builds offer several advantages:

  • Reduced Image Size: By copying only the necessary files from the build stage to the final image, you can minimize the size of the resulting Docker image.
  • Improved Build Efficiency: You can use larger base images for building without affecting the final image size.
  • Cleaner Dockerfiles: Multi-stage builds help keep your Dockerfile organized by separating the build and runtime environments.

2. How Multi-Stage Builds Work

In a multi-stage build, you define multiple FROM statements in your Dockerfile. Each FROM statement starts a new build stage. You can name these stages and selectively copy files from one stage to another using the COPY --from command.

Example: Multi-Stage Build for a Go Application

Here’s a simple example of a multi-stage build for a Go application:

FROM golang:1.16 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp

FROM alpine:latest
WORKDIR /root/
COPY --from=builder /app/myapp .
CMD ["./myapp"]

Explanation of the Example:

  • FROM golang:1.16 AS builder: This line specifies the first stage of the build, using the official Go image. The stage is named builder.
  • WORKDIR /app: Sets the working directory inside the container.
  • COPY . .: Copies the current directory contents into the container's working directory.
  • RUN go build -o myapp: Builds the Go application and outputs the binary named myapp.
  • FROM alpine:latest: This line starts a new stage using a minimal Alpine image.
  • WORKDIR /root/: Sets the working directory for the final image.
  • COPY --from=builder /app/myapp .: Copies the compiled binary from the builder stage to the final image.
  • CMD ["./myapp"]: Specifies the command to run when the container starts.

3. Building the Multi-Stage Docker Image

To build the Docker image using the above Dockerfile, run the following command:

docker build -t my_go_app .

4. Conclusion

Multi-stage builds in Docker are a powerful feature that allows you to create smaller, more efficient images by separating the build environment from the runtime environment. By using this approach, you can significantly reduce the size of your Docker images while maintaining a clean and organized Dockerfile. This practice is especially beneficial for applications that require a build process, such as those written in languages like Go, Java, or C.