How to Create a Docker Image: A Complete Guide
Docker images are the foundation of containerized applications. Whether you're packaging a web server, a machine learning pipeline, or a simple script, understanding how Docker images are built — and what shapes that process — is essential for anyone working with containers.
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, dependencies, configuration files, and application code. When you run an image, Docker creates a live instance called a container.
Images are built in layers. Each instruction in a build file adds a new layer on top of the previous one. Docker caches these layers, which means rebuilding an image after a small change only reprocesses the affected layers — not the entire image.
The Core Tool: The Dockerfile
The primary method for creating a Docker image is writing a Dockerfile — a plain-text file containing a sequence of instructions that Docker executes in order.
Essential Dockerfile Instructions
| Instruction | Purpose |
|---|---|
| FROM | Sets the base image (e.g., Ubuntu, Alpine, Python) |
| RUN | Executes shell commands during the build |
| COPY / ADD | Copies files from your local machine into the image |
| WORKDIR | Sets the working directory inside the image |
| ENV | Defines environment variables |
| EXPOSE | Documents which network port the container listens on |
| CMD / ENTRYPOINT | Defines the default command run when the container starts |
A minimal Dockerfile for a Python application might look like this: