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How to Create a Dockerfile: A Complete Guide
A Dockerfile is a plain-text script that tells Docker exactly how to build a container image. Every line is an instruction — from choosing a base operating system to copying your application files and defining how the container should start. Once you understand the structure, writing one becomes straightforward. But the right Dockerfile looks very different depending on what you're building and how you're deploying it.
What Is a Dockerfile, Exactly?
When Docker builds an image, it follows the Dockerfile step by step, creating a layered filesystem. Each instruction adds a new layer on top of the previous one. These layers are cached, which means Docker only rebuilds the layers that changed — making repeated builds significantly faster.
A container image built from a Dockerfile is portable and reproducible. The same image runs identically on a developer's laptop, a CI/CD pipeline, or a production server.
The Core Instructions You Need to Know
Here are the most commonly used Dockerfile instructions and what they do:
| Instruction | Purpose |
|---|---|
| FROM | Sets the base image (e.g., Ubuntu, Alpine, Node) |
| WORKDIR | Sets the working directory inside the container |
| COPY | Copies files from your local machine into the image |
| RUN | Executes a command during the build process |
| ENV | Sets environment variables |
| EXPOSE | Documents which port the container listens on |
| CMD | Defines the default command when the container starts |
| ENTRYPOINT | Sets a fixed executable that always runs |
Writing Your First Dockerfile 🐳
Here's a simple, real example for a Node.js web application: