Optimizing Docker Images for Size and Security: A Comprehensive Guide

Docker is a powerful tool that enables developers to containerize their applications and ensure consistency across various environments.

However, without careful consideration, Docker images can become bloated, slow, and vulnerable to security risks. In this guide, I’ll walk you through the strategies to optimize Docker images for both size and security, ensuring efficient and safe deployments.

Optimizing Docker Images for Size

The size of your Docker image directly affects how quickly it can be pulled and deployed, which will significantly reduce the pipeline run-time and artifact storage costs, so reducing the image size is crucial for performance and resource efficiency.

At the end of this section, I will show you my portfolio website's image size being reduced by almost 96%!

Here’s how you can minimize your image size:

1) Use Official Minimal Base Images

When building Docker images, always start with an official base image. Instead of using a full-sized OS image like ubuntu, opt for lightweight versions like alpine or debian-slim. These minimal images contain only the essentials, significantly reducing the image size.

Taking an example for node image, Here are the image sizes for node:latest vs node:alpine:

That's almost 7 times bigger !

By using minimal base images, you avoid unnecessary packages, leading to faster builds and smaller images.

2) Minimize Layers

Each instruction in your Dockerfile (RUN, COPY, etc.) creates a new layer in the final image. Combining related commands into a single layer reduces the number of layers and therefore the image size.

• Instead of doing this

RUN apt-get update
RUN apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

• Do this

RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

3) Exclude Unnecessary Files with '.dockerignore'

When building Docker images, Docker copies the entire context (everything in your project directory) into the image unless you specify otherwise. To prevent unnecessary files from being included, create a .dockerignore file.

• Example .dockerignore

node_modules
.git
logs
tmp

This file works similarly to .gitignore

4) Use Static Binaries and the 'scratch' Base Image

If your application can be compiled into a static binary, you can use the scratch base image, which is essentially an empty image. This leads to extremely small final images.

• Example

FROM scratch
COPY myapp /
CMD ["/myapp"]

Works well for applications that don’t need operating system-level dependencies.

5) Multi Stage Builds (Most Effective)

Multi-stage builds allow you to separate the build process from the runtime environment. This is especially useful when your application requires tools for compiling but doesn’t need them in the final image.

• Example

# Stage 1: Build
FROM golang:1.16-alpine AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp .

# Stage 2: Runtime
FROM alpine:latest
WORKDIR /app
COPY --from=builder /app/myapp .
CMD ["./myapp"]

Quantitative Comparison

My Portfolio Website which was built using React was previously built using node:14-alpine image which was still a smaller image than the node:latest image.

• The Dockerfile went like:

# Use an official Node runtime as a parent image
FROM node:14-alpine

# Set the working directory
WORKDIR /app

# Copy package.json and package-lock.json to the working directory
COPY package*.json ./

# Install dependencies
RUN npm install

# Copy the rest of the application code to the working directory
COPY . .

# Build the React app
RUN npm run build

# Install a lightweight HTTP server to serve the app
RUN npm install -g serve

# Set the default command to serve the build folder
CMD ["serve", "-s", "build"]

# Expose the port the app will run on
EXPOSE 3000

• The image built was of size:

Much later after this I learnt about Multi-Stage Builds and redesigned my Dockerfile.

• The new Dockerfile looked like:

# Build environment (Stage - I)
FROM node:14-alpine as build
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
RUN npm run build

# Production environment (Stage - II)
FROM nginx:alpine
COPY --from=build /app/build /usr/share/nginx/html
EXPOSE 80
CMD ["nginx", "-g", "daemon off;"]

Astonishingly, The new image size was ...

The application worked exactly as before and was much faster to spin up this version !

The difference created was of ~1079 MBs which is a decrease of almost 96% !

This is an illustration of the effect of Multi Stage Builds

Optimizing Docker Images for Security

1) Use Trusted and Official Base Images

Always use official base images from trusted sources like Docker Hub or your organization’s trusted registries. These images are regularly updated and are more secure compared to custom or unofficial images. Keep your base images up-to-date to mitigate any vulnerabilities.

2) Run Containers as Non-Root Users

Running containers as root can expose your host system to security risks. Create a non-root user inside the Dockerfile and configure your container to run under that user.

• Example:

RUN adduser --disabled-password myuser
USER myuser

Such simple change reduces the attack surface and improves security by limiting access to system resources.

3) Scan Images for Vulnerabilities

Regularly scan your Docker images for known vulnerabilities using tools like:

• Trivy: An open-source vulnerability scanner.
• Docker Scan: Built into the Docker CLI.
• Clair: A static analysis tool for discovering vulnerabilities.

These tools scan your images for outdated or insecure packages and alert you to potential threats.

4) Limit Network Exposure

Limit the network exposure of your container by restricting the ports and IP addresses it listens on. By default, Docker exposes ports to all interfaces. Bind them to localhost if external access is unnecessary.

• Example:

docker run -p 127.0.0.1:8080:8080 myimage

This restricts access to the container’s services to the local machine only, preventing external access.

4) Secrets Management

Avoid hardcoding sensitive information like API keys or passwords directly into your Dockerfile or environment variables. Instead, use Docker secrets or external secrets management tools like AWS Secrets Manager or HashiCorp Vault.

• Example Using Docker Secrets:

docker secret create my_secret secret.txt

Docker secrets ensure that sensitive data is only available to services that need it, without leaving traces in the container filesystem.

Conclusion

By following these strategies, you can build Docker images that are both lightweight and secure. Optimizing for size helps reduce deployment times, save resources, reduce costs and improve performance, while security best practices protect your application and infrastructure from vulnerabilities.

Remember, containerization offers many advantages, but it also introduces new challenges. With thoughtful image optimization, you can leverage Docker to its full potential while maintaining a robust security posture.

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Happy Learning !

Exclusively authored by,

👨‍💻 Akshat Gautam

Google Certified Associate Cloud Engineer | Full-Stack Developer

Feel free to connect with me on LinkedIn.