Skip to content

Introduction to Dockerfiles

A Dockerfile is a text file that contains instructions for building a Docker image. Think of it as a recipe that tells Docker exactly how to create a container image step by step. Each instruction in the Dockerfile adds a new "layer" to the image, making it a powerful way to create reproducible computing environments.

Key Concepts

Base Image: Every Dockerfile starts with a base image, specified by the FROM instruction. This is like choosing your starting point - it could be a minimal Linux distribution, or a pre-configured Python environment. The base image provides the foundation for your container.

Layers: Docker images are built in layers. Each instruction in your Dockerfile creates a new layer. These layers are cached, which means if you rebuild your image and nothing has changed in a particular layer, Docker will reuse the cached version, making builds faster.

Build Context: When you build a Docker image, Docker sends all the files in the current directory (and subdirectories) to the Docker daemon. This is called the build context. The .dockerignore file can be used to exclude files you don't want to include.

Common Instructions

Dockerfiles use a specific set of instructions, each serving a different purpose:

  • FROM - Sets the base image
  • WORKDIR - Sets the working directory
  • COPY and ADD - Copy files into the image
  • RUN - Executes commands
  • ENV - Sets environment variables
  • EXPOSE - Documents which ports the container will listen on
  • CMD - Sets the default command to run when starting a container

Best Practices

  1. Use specific base image versions to ensure reproducibility
  2. Minimize the number of layers for efficiency
  3. Place instructions that change frequently at the end of the Dockerfile
  4. Use a .dockerignore file to exclude unnecessary files
  5. Run applications as a non-root user for security
  6. Use multi-stage builds for smaller production images