A multi-stage Dockerfile is a technique used to optimize the size and performance of Docker images. It allows you to use multiple FROM statements in your Dockerfile, each defining a new stage of the build process. This way, you can separate the build environment from the final runtime environment, ensuring that your final image contains only the necessary components to run the application.

Here’s a clear presentation of a multi-stage Dockerfile with an example:

Example: Building a Go Application

Let's create a simple multi-stage Dockerfile for a Go application. This example will demonstrate how to compile a Go program and then create a smaller, final image with only the compiled binary.

Step-by-Step Explanation

1. First Stage - Build the Application:

   • Use a base image that has all the tools needed to build the application (e.g., golang).
   • Copy the source code and build the application.

2. Second Stage - Create the Final Image:

   • Use a minimal base image (e.g., alpine).
   • Copy only the necessary artifacts (e.g., the compiled binary) from the first stage.
   • Set the entrypoint to run the application.

Dockerfile

dockerfile
# First Stage: Build the application
FROM golang:1.18 AS builder

# Set the working directory inside the container
WORKDIR /app

# Copy the source code into the container
COPY . .

# Build the Go application
RUN go build -o myapp

# Second Stage: Create the final image
FROM alpine:latest

# Set the working directory inside the container
WORKDIR /app

# Copy the compiled binary from the builder stage
COPY --from=builder /app/myapp .

# Set the entrypoint to run the application
ENTRYPOINT ["./myapp"]

Explanation of Each Part

1. First Stage:

   • FROM golang:1.18 AS builder: Start a new build stage named builder using the golang image.
   • WORKDIR /app: Set the working directory to /app.
   • COPY . .: Copy all files from the current directory on the host to the /app directory in the container.
   • RUN go build -o myapp: Build the Go application and output the binary as myapp.

2. Second Stage:

   • FROM alpine:latest: Start a new build stage with a minimal alpine image.
   • WORKDIR /app: Set the working directory to /app.
   • COPY --from=builder /app/myapp .: Copy the myapp binary from the builder stage to the current directory in the alpine image.
   • ENTRYPOINT ["./myapp"]: Set the entrypoint to run the myapp binary.

Benefits of Multi-Stage Builds

• Smaller Final Images: Only the necessary files are included in the final image, reducing its size.
• Enhanced Security: By excluding build tools and other unnecessary files from the final image, the attack surface is minimized.
• Simplified Build Process: Different stages can be optimized independently, making the build process cleaner and more manageable.

Conclusion

Multi-stage builds in Dockerfiles provide a powerful way to optimize Docker images by separating the build environment from the runtime environment. This method ensures that your final Docker images are lean, secure, and efficient. The example provided demonstrates a typical use case in building a Go application, but the same principles can be applied to any application or environment.