Building a Docker Image¶

Overview¶

A Docker image is built using a Dockerfile, which defines the steps required to package an application.

Example Dockerfile¶

# Use base image
FROM node:18

# Set working directory
WORKDIR /app

# Copy files
COPY package.json .
COPY . .

# Install dependencies
RUN npm install

# Expose port
EXPOSE 3000

# Start application
CMD ["npm", "start"]

Build the Image¶

docker build -t my-app:1.0 .

Run the Container¶

docker run -p 3000:3000 my-app:1.0

How the Build Works¶

Docker builds images in layers:

• Base image is pulled
• Each instruction creates a new layer
• Layers are cached for efficiency

Image Layering Concept¶

Best Practices¶

• Use small base images (e.g. alpine)
• Minimise number of layers
• Use .dockerignore
• Avoid running as root
• Pin dependency versions

Common Mistakes¶

• Copying unnecessary files into image
• Not using caching effectively
• Large image sizes

Real-World Implications¶

• Smaller images = faster deployments
• Efficient builds reduce CI/CD time
• Clean images reduce security risks

Key Takeaways¶

• Dockerfiles define how images are built
• Images are layered and cached
• Good practices improve performance and security