How to Create a Docker Image: A Practical Guide
Docker images are the foundation of containerized applications. Whether you're packaging a web app, a microservice, or a data pipeline, knowing how to build a Docker image is a core skill in modern software development and deployment. Here's what the process actually involves — and why the right approach varies considerably depending on your setup.
What Is a Docker Image?
A Docker image is a read-only template that contains everything needed to run an application: the operating system layer, runtime environment, application code, libraries, and configuration. When you run an image, Docker creates a live container from it.
Images are built in layers. Each instruction in your build file adds a layer on top of the previous one. Docker caches these layers, which means rebuilds are often fast — only the layers that changed get rebuilt.
Images are stored locally on your machine or pushed to a container registry (like Docker Hub, Amazon ECR, or GitHub Container Registry) so others can pull and run them.
The Core Tool: The Dockerfile
The primary way to create a Docker image is by writing a Dockerfile — a plain text file with a specific set of instructions that Docker reads top to bottom.
Essential Dockerfile Instructions
| Instruction | What It Does |
|---|---|
| FROM | Sets the base image (e.g., ubuntu, node, python) |
| WORKDIR | Sets the working directory inside the container |
| COPY / ADD | Copies files from your host machine into the image |
| RUN | Executes a command during the build (e.g., install packages) |
| ENV | Sets environment variables |
| EXPOSE | Documents which port the app listens on |
| CMD / ENTRYPOINT | Defines the default command when the container starts |
A minimal example for a Python web app might look like this: