Dockerfile Best Practices

Creating a Dockerfile is a critical part of building container images. Following best practices not only leads to optimized images but also enhances security and maintainability. The following are some best practices to consider when writing your Dockerfile.

Start with a Small Base Image

Use minimal base images like alpine or distroless when possible. This reduces the attack surface and the overall image size.

Use Official Images

Whenever possible, start with official images from Docker Hub. These images are usually maintained and updated regularly.

Write Layer-Friendly Commands

Combine commands where possible using &&. Each command in a Dockerfile creates a new layer. Reduced layers mean smaller image size and faster build times.

RUN apt-get update && apt-get install -y package1 package2 && rm -rf /var/lib/apt/lists/*

Order Instructions from Least to Most Frequently Changing

Place the instructions that are least likely to change at the top of your Dockerfile (e.g., installing dependencies), and the instructions that do change more often towards the bottom (e.g., application source code). This way, Docker caches layers effectively.

Utilize Multi-Stage Builds

Use multi-stage builds to keep the final image light. Build your application in one stage, and then copy only the artifacts you need to the final image.

FROM golang:alpine AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp

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

Set the Working Directory

Use WORKDIR to create a consistent working directory for your commands, avoiding the need to repeatedly specify paths.

Avoid Using latest Tag

Specify exact versions of images for predictability and security. Using latest can lead to unexpected changes.

Use .dockerignore

Create a .dockerignore file to exclude files and directories that should not be copied into the image. This keeps your image size down and enhances security.

Limit the Use of RUN Commands

Be careful with RUN apt-get install because it can leave behind unnecessary files. Always clean up after installations.

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

Security Considerations

Run containers as a non-root user:

RUN useradd -m myuser
USER myuser

Avoid hardcoding sensitive information directly into a Dockerfile; use Docker secrets or environment variables.

Use ENTRYPOINT vs. CMD

Prefer ENTRYPOINT for the main command of the container and CMD for providing default arguments to it. This allows users to override or extend commands easily.

ENTRYPOINT ["python"]
CMD ["app.py"]

Regularly Update Dependencies

Keep your base images and dependencies updated to the latest versions to ensure you are leveraging improvements and security fixes.

Conclusion

By adhering to these best practices, you can create Dockerfiles that are efficient, easy to maintain, and secure. This will help ensure that your applications run smoothly and reliably in a containerized environment.