Understanding Docker Images and Containers

Now let's see the difference between a docker image & a docker container.

Note that Docker container and Docker images are two separate things and are not interchangeable in any means. Let’s go through the basic definitions & differences of both.

Docker Images

• Docker Image is a combination of file system and parameters & you can think of it as a single package that holds up everything you need to run your application. You can also refer a docker image as a MAGIC file.
• A Docker image doesn’t have any state attached to it and once built it never changes.
• A Docker image is something that you can download, build and run.
• You can also consider a Docker image as a result of running a series of steps.

Docker Containers

• On the contrary, The act of running a Docker image is called container. In developer’s perspective, you can think of a Docker image as a class and Docker container as an instance of the class. Like first you define a class then you create an instance of it.
• You can launch many containers from a single image. For example if you’ve a single image of your web application
• Docker Containers are immutable, means that any changes you make on them while running will be lost forever when you stop that container.

For example, let’s run a simple linux container by:

docker run -it alpine sh

and make a new folder in the home directory and exit and rebuild the container and you’ll see there’s no such file in the home directory. Hence Docker containers are immutable.