Dockerfiles – best practices

Introduction:

Docker has revolutionized the way developers deploy and manage applications. With Docker, developers can package their applications into portable containers, which can be run anywhere, without worrying about the underlying infrastructure. One of the key components of Docker is the Dockerfile, which is used to define the contents of a Docker image. In this blog post, we will discuss some best practices for writing Dockerfiles.

1. Use a minimal base image:
   One of the most important Dockerfile best practices is to use a minimal base image. Starting with a minimal base image such as Alpine Linux can significantly reduce the size of the final Docker image, which can lead to faster image downloads and improved performance. Here is an example of a Dockerfile that uses Alpine Linux as the base image:

FROM alpine:3.14.2

2. Use official images:
   Whenever possible, it is recommended to use official images from Docker Hub as the base image. Official images are maintained by the community and are regularly updated to include security patches and bug fixes. Here is an example of a Dockerfile that uses an official image as the base image:

FROM node:14-alpine

3. Avoid running as root:
   Running containers as root can be a security risk. It is recommended to use a non-root user for running the container, and set appropriate file permissions. Here is an example of a Dockerfile that creates a non-root user:

FROM node:14-alpine

RUN addgroup -S appgroup && adduser -S appuser -G appgroup
USER appuser

4. Minimize layers:
   Reducing the number of layers in a Dockerfile can lead to faster builds and smaller image sizes. This can be achieved by combining multiple commands into a single layer using the && operator. Here is an example of a Dockerfile that combines multiple commands into a single layer:

FROM node:14-alpine

RUN apk update && \
    apk add git && \
    git clone https://github.com/example/myapp.git /app

5. Cache dependencies:
   Docker has a caching mechanism that can be used to cache dependencies. This can significantly speed up the build process by avoiding downloading dependencies every time the image is built. Here is an example of a Dockerfile that caches dependencies:

FROM node:14-alpine

WORKDIR /app
COPY package.json package-lock.json ./
RUN npm install --production
COPY . .

CMD ["npm", "start"]

6. Use COPY instead of ADD:
   The COPY instruction is preferred over ADD because it is simpler and less prone to unexpected behavior. Here is an example of a Dockerfile that uses COPY:

FROM node:14-alpine

WORKDIR /app
COPY package.json package-lock.json ./
RUN npm install --production
COPY . .

CMD ["npm", "start"]

7. Keep the image up-to-date:
   Regularly updating the base image and dependencies is important to ensure that the image is up-to-date and secure. Here is an example of a Dockerfile that updates the base image and dependencies:

FROM node:14-alpine

RUN apk update && apk upgrade && \
    apk add git && \
    git clone https://github.com/example/myapp.git /app

WORKDIR /app
RUN npm install --production

CMD ["npm", "start"]

8. Remove unnecessary files:
   Removing any unnecessary files or directories can help reduce the size of the final Docker image. Here is an example of a Dockerfile that removes unnecessary files:

FROM node:14-alpine

RUN apk update && apk upgrade && \
apk add git && \
git clone https://github.com/example/myapp.git /app

9. Use environment variables:
   Using environment variables for configuration instead of hardcoding values in the Dockerfile can make the image more flexible and easier to maintain. Here is an example of a Dockerfile that uses environment variables:

FROM node:14-alpine

ENV NODE_ENV=production

WORKDIR /app
COPY package.json package-lock.json ./
RUN npm install
COPY . .

CMD ["npm", "start"]

In this example, we set an environment variable called NODE_ENV to production using the ENV instruction. This variable can be used by the application code to determine its environment.

10. Document the Dockerfile:
    Providing clear documentation in the Dockerfile can help other developers understand how the image was built and how it should be used. Here is an example of a Dockerfile with clear documentation:

# Use a minimal base image
FROM alpine:3.14.2

# Install required packages
RUN apk add --no-cache python3

# Set the working directory
WORKDIR /app

# Copy the application code
COPY . /app

# Install dependencies
RUN pip3 install -r requirements.txt

# Expose the default port
EXPOSE 5000

# Set the default command
CMD ["python3", "app.py"]

In this example, we provide comments to explain what each instruction does, and we also explain how the image should be used. This can help other developers who may need to modify or extend the Dockerfile.

Conclusion:
These are the nine best practices for writing Dockerfiles. Following these guidelines can help you create Docker images that are efficient, secure, and easy to maintain. By adopting these best practices, you can ensure that your Docker images are built to the highest standards and are ready to be deployed to production environments.