Copying from build stage to final stage in Docker - Mini Project: Build & Apply

Copying from build stage to final stage

📖 Scenario: You are creating a Docker image for a simple web application. To keep the final image small, you want to use a multi-stage build. The first stage will build the application, and the second stage will copy only the necessary files from the build stage.

🎯 Goal: Build a multi-stage Dockerfile where the final stage copies the built application files from the build stage.

📋 What You'll Learn

Create a build stage named builder using the node:18 image

In the build stage, create a directory /app and add a file index.js with the content console.log('Hello from build stage');

Create a final stage using the node:18-slim image

Copy the index.js file from the builder stage to the final stage's /app directory

Set the working directory to /app in the final stage

Set the default command to run node index.js

💡 Why This Matters

🌍 Real World

Multi-stage builds are used in real projects to separate building and running environments. This keeps the final Docker image small and secure by excluding build tools.

💼 Career

Understanding multi-stage Docker builds is important for DevOps roles, as it improves deployment efficiency and image management.

1

Create the build stage

Create a build stage named builder using the node:18 image. Inside it, create a directory /app and add a file index.js with the content console.log('Hello from build stage'); using a RUN command.

Docker

# Create build stage named builder with node:18
# Create /app directory and add index.js file with console.log message

Need a hint?

Use FROM node:18 AS builder to name the build stage. Use RUN mkdir /app to create the directory. Use echo to create the index.js file with the exact content.

2

Create the final stage

Create the final stage using the node:18-slim image. This stage will be the last FROM instruction in the Dockerfile.

Docker

FROM node:18 AS builder
RUN mkdir /app && echo "console.log('Hello from build stage');" > /app/index.js

# Create final stage using node:18-slim

Need a hint?

Use FROM node:18-slim to start the final stage.

3

Copy files from build stage to final stage

In the final stage, copy the index.js file from the builder stage's /app directory to the final stage's /app directory using the COPY --from=builder command. Then set the working directory to /app.

Docker

FROM node:18 AS builder
RUN mkdir /app && echo "console.log('Hello from build stage');" > /app/index.js

FROM node:18-slim
# Copy index.js from builder stage to /app
# Set working directory to /app

Need a hint?

Use COPY --from=builder /app/index.js /app/index.js to copy the file. Use WORKDIR /app to set the working directory.

4

Set the default command

In the final stage, add a CMD instruction to run node index.js when the container starts.

Docker

FROM node:18 AS builder
RUN mkdir /app && echo "console.log('Hello from build stage');" > /app/index.js

FROM node:18-slim
COPY --from=builder /app/index.js /app/index.js
WORKDIR /app
# Set default command to run node index.js

Need a hint?

Use CMD ["node", "index.js"] to set the default command.