Why multi-stage builds reduce image size in Docker - See It in Action

Why Multi-Stage Builds Reduce Image Size

📖 Scenario: You are working on a Docker project where you want to create a small and efficient container image for a simple Go application. You want to learn how multi-stage builds help reduce the final image size by separating build and runtime environments.

🎯 Goal: Build a Dockerfile using multi-stage builds that compiles a Go program in one stage and copies only the compiled binary to a smaller base image in the final stage. This will show how multi-stage builds reduce image size.

📋 What You'll Learn

Create a Dockerfile with two stages: a build stage and a final stage

Use golang:1.20 as the build stage base image

Compile a Go program called hello.go in the build stage

Use alpine:latest as the final stage base image

Copy only the compiled binary from the build stage to the final stage

Set the container to run the compiled binary

💡 Why This Matters

🌍 Real World

Developers often need to create small, efficient Docker images to save bandwidth and speed up deployment.

💼 Career

Understanding multi-stage builds is important for DevOps roles to optimize container images and improve CI/CD pipelines.

1

Create the Go source file

Create a file named hello.go with a simple Go program that prints "Hello, Docker!" when run. The program should have a main package and a main function with fmt.Println("Hello, Docker!").

Docker

# Create hello.go with a simple Go program
# Your code here

Hint

Use package main and import fmt. The main function should print the message.

2

Set up the build stage in Dockerfile

In a file named Dockerfile, create the first stage called builder using the base image golang:1.20. Copy the hello.go file into the container at /app/hello.go and set the working directory to /app. Add a command to build the Go program into a binary named hello.

Docker

package main

import "fmt"

func main() {
    fmt.Println("Hello, Docker!")
}

# Start Dockerfile with build stage
# Your code here

Hint

Use FROM golang:1.20 AS builder to name the build stage. Use WORKDIR and COPY to prepare files. Use RUN go build -o hello hello.go to compile.

3

Add the final stage to copy the binary

In the same Dockerfile, add a final stage using the base image alpine:latest. Copy the compiled binary hello from the builder stage to /hello in the final image. Set the container to run /hello when started.

Docker

package main

import "fmt"

func main() {
    fmt.Println("Hello, Docker!")
}

# Dockerfile
FROM golang:1.20 AS builder
WORKDIR /app
COPY hello.go .
RUN go build -o hello hello.go

# Add final stage below
# Your code here

Hint

Use FROM alpine:latest for the final stage. Copy the binary with COPY --from=builder. Use CMD ["/hello"] to run the program.

4

Build and run the Docker image to see the output

Build the Docker image with the tag hello-multistage using the command docker build -t hello-multistage .. Then run the container with docker run --rm hello-multistage and observe the output.

Docker

package main

import "fmt"

func main() {
    fmt.Println("Hello, Docker!")
}

# Dockerfile
FROM golang:1.20 AS builder
WORKDIR /app
COPY hello.go .
RUN go build -o hello hello.go

FROM alpine:latest
COPY --from=builder /app/hello /hello
CMD ["/hello"]

# Build and run commands
# Your code here

Hint

Use docker build -t hello-multistage . to build and docker run --rm hello-multistage to run the container.

Practice

1. Why do multi-stage builds in Docker help reduce the final image size?

easy

A. Because they run containers faster

B. Because they use a single large base image for all stages

C. Because they copy only the necessary files from build stages to the final image

D. Because they install all dependencies in every stage

Solution

1. Step 1: Understand multi-stage build process

   Multi-stage builds use multiple FROM statements to separate build and runtime environments.

2. Step 2: Identify file copying behavior

   Only necessary artifacts are copied from build stages to the final image, excluding build tools and dependencies.

3. Final Answer:

   Because they copy only the necessary files from build stages to the final image -> Option C

4. Quick Check:

   Copying only needed files = smaller image [OK]

Hint: Remember: copy only what you need to keep image small [OK]

Common Mistakes:

• Thinking all stages are combined fully
• Assuming all dependencies stay in final image
• Believing multi-stage builds speed up container runtime

2. Which of the following is the correct syntax to copy files from a previous stage named builder in a multi-stage Dockerfile?

easy

A. COPY --from=builder /app/build /app

B. COPY builder:/app/build /app

C. COPY /app/build builder:/app

D. COPY --stage=builder /app/build /app

Solution

1. Step 1: Recall Dockerfile COPY syntax for multi-stage

   The correct syntax uses COPY --from=stageName source destination.

2. Step 2: Match syntax with options

   COPY --from=builder /app/build /app matches the correct syntax exactly.

3. Final Answer:

   COPY --from=builder /app/build /app -> Option A

4. Quick Check:

   Use --from=stageName to copy files [OK]

Hint: Use COPY --from=stageName to copy files between stages [OK]

Common Mistakes:

• Using incorrect flags like --stage
• Trying to copy with colon syntax like builder:/path
• Reversing source and destination paths

3. Given this Dockerfile snippet, what will be the size impact on the final image?

FROM node:18 AS builder
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
RUN npm run build

FROM nginx:alpine
COPY --from=builder /app/build /usr/share/nginx/html

medium

A. The final image will be small because only the build output is copied

B. The final image will be empty because no files are copied

C. The final image will be large because it includes all node modules

D. The final image will include the entire node base image

Solution

1. Step 1: Analyze build and final stages

   The builder stage installs node modules and builds the app. The final stage uses a lightweight nginx image.

2. Step 2: Check what is copied to final image

   Only the build output folder /app/build is copied to nginx's html folder, excluding node modules and build tools.

3. Final Answer:

   The final image will be small because only the build output is copied -> Option A

4. Quick Check:

   Copying only build output reduces image size [OK]

Hint: Copy only build artifacts, not full build environment [OK]

Common Mistakes:

• Assuming node modules are copied to final image
• Thinking final image includes builder base image
• Believing no files are copied at all

4. You wrote a multi-stage Dockerfile but the final image is unexpectedly large. Which mistake could cause this?

medium

A. Using a lightweight base image in the final stage

B. Using multiple FROM statements

C. Not naming the build stage

D. Copying the entire build directory including node_modules to the final stage

Solution

1. Step 1: Identify common causes of large final images

   Copying unnecessary files like dependencies increases image size.

2. Step 2: Match mistake to options

   Copying the entire build directory including node_modules to the final stage describes copying the entire build directory including node_modules, which bloats the image.

3. Final Answer:

   Copying the entire build directory including node_modules to the final stage -> Option D

4. Quick Check:

   Copying unneeded files causes large images [OK]

Hint: Avoid copying full build folders with dependencies [OK]

Common Mistakes:

• Thinking multiple FROM statements cause large images
• Believing missing stage names cause size issues
• Assuming lightweight base images increase size

5. You want to build a Go application using multi-stage builds. The first stage compiles the app, and the second stage runs it. Which Dockerfile snippet best reduces the final image size?

hard

A. FROM golang:1.20
WORKDIR /app
COPY . .
RUN go build -o app
CMD ["./app"]

B. FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o app

FROM alpine
COPY --from=builder /app/app /app/app
CMD ["/app/app"]

C. FROM alpine
COPY . .
RUN go build -o app
CMD ["./app"]

D. FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o app
COPY . .
CMD ["./app"]

Solution

1. Step 1: Understand multi-stage build for Go app

   Build the app in a full Go environment, then copy only the compiled binary to a small base image.

2. Step 2: Evaluate options for size reduction

   FROM golang:1.20 AS builder WORKDIR /app COPY . . RUN go build -o app FROM alpine COPY --from=builder /app/app /app/app CMD ["/app/app"] uses golang image to build, then copies only the binary to alpine, minimizing final image size.

3. Final Answer:

   FROM golang:1.20 AS builder WORKDIR /app COPY . . RUN go build -o app FROM alpine COPY --from=builder /app/app /app/app CMD ["/app/app"] -> Option B

4. Quick Check:

   Build in full image, run in minimal image [OK]

Hint: Build in full image, copy only binary to small image [OK]

Common Mistakes:

• Building and running in same large image
• Trying to build in alpine without Go tools
• Copying full source code to final image