Docker has become an essential tool in modern software development, enabling developers to build, package, and deploy applications in lightweight, portable containers. A Dockerfile is the blueprint for creating a Docker image, and how you write it can significantly affect the efficiency, security, and maintainability of your application.
In this blog post, we’ll discuss best practices for writing efficient Dockerfiles. By following these practices, you can optimize the build process, improve container performance, and make your Dockerfiles easier to maintain.
Why Efficient Dockerfiles Matter
An efficient Dockerfile leads to:
• Smaller Docker images: This means faster builds, quicker deployments, and reduced storage and bandwidth usage.
• Faster build times: Efficient Dockerfiles minimize the time it takes to build and rebuild containers.
• Better security: Best practices help avoid unnecessary vulnerabilities and minimize the attack surface of your container.
• Maintainability: Clear and clean Dockerfiles are easier to manage and modify in the long run.
Best Practices for Writing Efficient Dockerfiles

1. Start with a Minimal Base Image
   Choosing the right base image is crucial. A minimal base image reduces the size of your Docker image and ensures that it only contains the essentials.
   • Use official images: Whenever possible, start with a well-maintained and official base image from Docker Hub. For example, use python:3.9-slim instead of python:3.9 to avoid unnecessary bloat.
   • Prefer Alpine Linux: If your application doesn’t have specific dependencies, consider using Alpine Linux-based images (alpine) as they are much smaller than other distributions. However, be aware that Alpine may not include all libraries, so you may need to install them manually.
   Example:
   FROM node:16-alpine
   This image is minimal and optimized for size, reducing the footprint of your container.
2. Leverage Caching by Ordering Instructions Wisely
   Docker builds images in layers, and each layer is cached unless the instruction changes. This means that the order in which you write instructions in the Dockerfile is important. By strategically ordering commands, you can take full advantage of Docker’s caching mechanism to speed up builds.
   • Install dependencies first: Separate installing dependencies (such as RUN apt-get install or RUN pip install) from copying your application’s code. This way, Docker will cache the dependency layer, and only the application layer will need to be rebuilt if code changes.
   Example:

Install dependencies first to leverage Docker caching

COPY requirements.txt /app/requirements.txt
RUN pip install –no-cache-dir -r /app/requirements.txt

Now copy the rest of the application code

COPY . /app
In this example, if your application code changes frequently but dependencies remain the same, only the last step will be re-executed, saving time.

3. Minimize the Number of Layers
   Each Dockerfile instruction (like RUN, COPY, and ADD) creates a new image layer. Too many layers can increase the size of your image and slow down the build process.
   • Combine multiple commands into a single RUN statement: For example, instead of running RUN apt-get update and RUN apt-get install separately, combine them into one command.
   Example:

Less efficient: Multiple layers created

RUN apt-get update
RUN apt-get install -y curl

More efficient: Single layer created

RUN apt-get update && apt-get install -y curl
Using && to chain commands into one RUN statement reduces the number of layers, making your Docker image more efficient.

4. Avoid Using ADD Unless Necessary
   The ADD instruction in Dockerfiles automatically handles compressed files and remote URLs. However, if you don’t need those special features, use COPY instead, as it’s simpler, more predictable, and doesn’t have the overhead of managing remote files or decompression.
   • Use COPY when you’re just copying files from your local context into the image.
   • Use ADD only when you need its extra capabilities (like extracting tar files or downloading remote URLs).
   Example:

Use COPY to copy local files

COPY . /app

Only use ADD for remote URLs or extracting archives

ADD https://example.com/file.tar.gz /app

5. Clean Up After Installation
   Sometimes, when you install dependencies, the package manager leaves behind unnecessary files, such as cache files. To keep the image clean and small, make sure to clean up after installing packages.
   • Remove package manager caches: For example, after installing system packages, remove package manager cache files to reduce image size.
   Example:
   RUN apt-get update && apt-get install -y \
   curl \
   && rm -rf /var/lib/apt/lists/*
   This ensures that the package manager cache is removed after installation, reducing the image size.
6. Use .dockerignore to Exclude Unnecessary Files
   Just like a .gitignore file, a .dockerignore file helps you exclude files and directories from the build context, ensuring they aren’t copied into the Docker image. This can significantly reduce image size and speed up the build process.
   • Add files like node_modules/, .git/, *.log, and other unnecessary files to .dockerignore to avoid copying them into your image.
   Example: .dockerignore
   node_modules/
   .git/
   *.log
   *.md
   By ignoring unnecessary files, you ensure that Docker only copies what’s needed, speeding up the build and keeping your image lean.
7. Set the Correct Working Directory
   The WORKDIR instruction sets the working directory inside the container. It’s a good practice to define a clear, specific working directory for your application code.
   • Avoid using arbitrary paths. Instead, use a directory like /app or /usr/src/app to keep things organized.
   Example:
   WORKDIR /app
   COPY . .
   This ensures that your application files are placed in a dedicated folder within the container.
8. Limit Container Privileges
   When running your application, it’s a good idea to minimize its privileges for better security. Docker allows you to set user permissions inside the container, ensuring the app doesn’t run as the root user.
   • Use the USER instruction to specify a non-root user to run the application.
   Example:

Create a non-root user and set it as the default user for the container

RUN useradd -m myuser
USER myuser
Running your container with a non-root user enhances security, particularly if your container is deployed in a shared or public environment.

9. Use Multi-Stage Builds
   Multi-stage builds allow you to create Docker images in stages, which can help you separate the build environment from the runtime environment. This helps create smaller, more secure production images by excluding unnecessary build dependencies from the final image.
   • Build in one stage, then copy only the necessary artifacts to the final image.
   Example:

Build stage

FROM node:16-alpine as build
WORKDIR /app
COPY . .
RUN npm install

Production stage

FROM node:16-alpine
WORKDIR /app
COPY –from=build /app /app
CMD [“node”, “app.js”]
In this example, the build stage installs dependencies, while the final stage only copies over the compiled application, resulting in a much smaller production image.
Conclusion
Writing efficient Dockerfiles is a key skill that can help you optimize your Docker images, streamline your build process, and improve container performance. By following the best practices outlined above—such as starting with a minimal base image, leveraging caching, cleaning up after installations, and using multi-stage builds—you can create Dockerfiles that are both effective and efficient.
With these best practices in hand, you’ll be well on your way to writing Dockerfiles that not only perform well but are also easier to maintain and secure. Happy Dockerizing!