Docker image vs container: the key difference
These two words get mixed up all the time, so let's make the difference crystal clear.
- An image is a read-only template: a packaged snapshot of an app plus everything it needs (files, libraries, settings). It doesn't run - it just sits there, ready.
- A container is a running instance of an image. When you start an image, you get a container.
Image vs container: a simple analogy
Think of an image like a recipe, and a container like a meal you cook from it. From one recipe you can cook the same meal many times. From one image you can start many containers, and they'll all be identical to begin with.
Another common analogy for programmers: an image is like a class, and a container is like an object (an instance) created from that class. One class, many objects.
Why this matters
When you ran docker run hello-world in
the previous chapter, Docker took the
hello-world image and created a container from it. If you ran the command
again, you'd get a second, separate container from the same image.
This separation is powerful:
- Images are shareable and reusable. You build one and run it anywhere.
- Containers are disposable. You can start them, stop them, throw them away, and start fresh - the image is untouched.
Keep this in mind for the rest of the course:
Image = the template (doesn't run). Container = a running copy of that template.
Next, let's practice running containers and controlling them.
Where this trips people up
The confusion usually comes from the word "run". When you edit files inside a running container and then throw that container away, your changes go with it - the image never changed. To actually change what every future container contains, you rebuild the image (a later chapter). Beginners often expect edits inside a container to "stick"; they don't, and that's by design.
FAQ
Can one image have multiple containers?
Yes. An image is just a template, so you can start as many containers from it as you like, and they all begin identical. This is exactly how you run several copies of the same app.
If I change a file inside a container, does the image change?
No. Changes live in that one container. The image stays untouched, and a new container from that image starts clean. To bake changes into the image, you rebuild it.
Which comes first, the image or the container?
The image. You build or download an image first, then create containers from it. No image, no container.