What is Docker multi-stage build?

What is Docker Multi-Stage Build?

Docker's multi-stage build is a feature that allows you to create smaller and more efficient Docker images by breaking down the build process into multiple stages. This is particularly useful when you have a complex build process that involves compiling code, installing dependencies, and performing other tasks.

In a traditional Docker build process, you would typically have a single Dockerfile that includes all the steps required to build your application. This can result in a large and bloated Docker image, which can be slow to build and deploy. With multi-stage builds, you can break down the build process into multiple stages, each with its own base image and set of instructions.

Here's how it works:

1. Base Image: This is the starting point for your build process, typically a lightweight base image like alpine or scratch.
2. Build Stage: In this stage, you perform the tasks required to build your application, such as compiling code, installing dependencies, and running tests. This stage can use a larger and more feature-rich base image, such as golang or node.
3. Release Stage: In this stage, you copy the built artifacts from the previous stage and package them into a smaller and more optimized final image. This stage typically uses a lightweight base image, such as alpine.
4. Final Image: The final Docker image that you can use to deploy your application.

By using a multi-stage build, you can significantly reduce the size of your final Docker image, which can improve the speed of your build and deployment processes. Additionally, you can keep your sensitive build-time dependencies and tooling isolated from the final image, improving the security and maintainability of your application.

Here's an example of a multi-stage Dockerfile for a simple Go application:

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

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

In this example, the first stage (builder) uses the golang:1.16 base image to compile the Go code and create the myapp binary. The second stage (release) then copies the myapp binary from the previous stage and packages it into a smaller alpine:latest base image.

By using a multi-stage build, we can ensure that the final Docker image only contains the necessary runtime components, without the build-time dependencies and tooling. This results in a smaller and more efficient Docker image that is faster to build and deploy.