Docker is a powerful tool that simplifies application deployment by packaging everything into isolated environments called containers. But if you’re new to Docker, you might wonder: what’s the difference between a Docker image and a Docker container?
Although these terms are often used together, they refer to two distinct components of the Docker ecosystem. In this post, we’ll break down their definitions, differences, and how they work together in the container lifecycle.
📦 What Is a Docker Image?
A Docker image is a read-only blueprint used to create containers. It includes:
- The application code
- Runtime (like Node.js, Python, Java, etc.)
- System tools and libraries
- Environment variables and dependencies
Think of a Docker image as a template—a static file that defines how your app should run.
✅ Key Properties of Images:
- Immutable (cannot be changed once built)
- Layered (built from instructions in a
Dockerfile) - Reusable (can be pushed to Docker Hub or other registries)
- Versioned (tagged with names like
myapp:latest)
🧱 What Is a Docker Container?
A Docker container is a running instance of a Docker image. It’s a lightweight, isolated, and executable environment where your app actually runs.
You can start, stop, restart, and destroy containers without affecting the underlying image.
✅ Key Properties of Containers:
- Mutable (can store runtime state)
- Isolated (each container has its own filesystem, processes, and network)
- Ephemeral (by default, data is lost when stopped or deleted unless volumes are used)
🧠 Real-World Analogy
| Concept | Analogy |
|---|---|
| Docker Image | A class in programming or a recipe |
| Docker Container | An object or an actual cooked dish |
An image is the instructions, while a container is the live, running result of following those instructions.
🔧 Workflow: From Image to Container
- Write a Dockerfile
- Build an image using:
docker build -t myapp:latest . - Run a container from the image:
docker run -d --name my-container myapp:latest
🧪 Side-by-Side Comparison
| Feature | Docker Image | Docker Container |
|---|---|---|
| State | Static (read-only) | Dynamic (runtime, can change) |
| Purpose | Blueprint for creating containers | Actual execution environment |
| Lifecycle | Built once, reused many times | Created from image, runs independently |
| Persistence | Stored in registries or locally | Runs temporarily (unless persisted) |
| Example | python:3.10-slim | A running Python app using that image |
🛠 Commands to Work With Images and Containers
🔹 List Images:
docker images
🔹 List Containers:
docker ps -a
🔹 Remove an Image:
docker rmi image-name
🔹 Stop and Remove a Container:
docker stop container-name
docker rm container-name
✅ Summary
| Term | What It Is | When to Use |
|---|---|---|
| Docker Image | A read-only template | To define your application environment |
| Container | A live, isolated instance | To run your application |
🚀 Final Thoughts
Understanding the distinction between images and containers is crucial to mastering Docker. Images define the “what” of your application, while containers define the “how” and “where” it runs.
💡 Pro tip: One image can be used to start many containers—each one isolated and customizable.