What is the difference between a Docker image and a container?
The image is the blueprint
A Docker image is a packaged snapshot of an app with its runtime, libraries and configuration. You build images from a Dockerfile or pull them from a registry such as Docker Hub. Images are stored as layers, so Docker can reuse layers that have not changed.
Images do not run on their own. They sit on disk until you start them, and running them does not change the image itself.
The container is a running copy
A container is what you get when Docker starts an image. It adds a writable layer on top of the image, plus its own network and process space. Changes made inside a running container stay in that layer and disappear when the container is removed, unless you use a volume.
- Image: the template, built once
- Container: a running or stopped instance
- Many containers can come from one image
- Tags such as myapp:1.2 identify image versions
Why it matters for deployment
Deployments often build an image once, then run containers from it in each environment. That makes rollbacks easier, because you can start a container from an older image tag.
Common mistakes
- Treating a running container as the source of truth instead of rebuilding the image.
- Storing data in a container writable layer and losing it on removal.
- Using only the latest tag, which makes it hard to know which version is running.
