Removing Unused Images in Docker

In this article, you can get training on the effective methods for managing Docker images, specifically focusing on removing unused images. Docker has revolutionized the way we build, ship, and run applications, but with the convenience of containerization comes the challenge of managing disk space. Unused images can accumulate over time, leading to wasted resources and slowed performance. Let’s dive into the practical steps for identifying and removing these unused images to keep your Docker environment clean and efficient.

Identifying Unused Images

Before we can remove unused images, it’s important to understand how to identify them. In a Docker environment, an image is considered unused if it is not associated with any running or stopped containers. This can occur when you build new images or pull images from a registry without removing the old ones.

To list all Docker images on your system, you can use the following command:

docker images

This command provides a snapshot of all images, including their repository, tag, image ID, and creation date. You’re primarily interested in images with no associated containers. To find these, you can utilize the docker ps command to list running and stopped containers:

docker ps -a

By cross-referencing the output of these two commands, you can identify images that are not being used. For example, consider the output of the following commands:

$ docker images
REPOSITORY          TAG       IMAGE ID       CREATED         SIZE
my-flask-app       latest    abcdef123456   3 days ago      50MB
unused-image       latest    123456abcdef   5 days ago      45MB

$ docker ps -a
CONTAINER ID   IMAGE         COMMAND                  CREATED        STATUS                     PORTS      NAMES
1a2b3c4d5e6f   my-flask-app  "python app.py"         3 days ago    Exited (0) 2 days ago                 my-flask

In this example, unused-image has no associated containers and is therefore a candidate for removal.

Using the Remove Image Command

Once you’ve identified the unused images, the next step is to remove them. Docker provides a straightforward command for this purpose:

docker rmi <image_id>

For instance, to remove the unused image identified in the previous section, you would run:

docker rmi 123456abcdef

However, you may encounter issues if the image is being used by a stopped container. In such cases, you can force the removal of an image using the -f flag:

docker rmi -f 123456abcdef

It’s worth noting that removing images can sometimes lead to orphaned containers that are no longer associated with any image. If you wish to clean up these containers as well, you can follow up with the docker rm command.

Example of Complete Image Removal Process

Let’s go through a complete example where you build a Python Flask application, identify unused images, and remove them.

• Build your Docker image using the provided Dockerfile:

docker build -t my-flask-app .

• List all images:

docker images

• Identify unused images by checking against running/stopped containers.
• Remove the unused image:

docker rmi <image_id>

By following these steps, you can keep your Docker environment organized and free from unnecessary clutter.

Cleaning Up with Prune

For a more comprehensive cleanup, Docker provides the prune command, which allows you to remove not only unused images but also stopped containers, unused networks, and dangling build cache. This is a powerful command, so it should be used with caution.

To remove all unused Docker objects, you can execute:

docker system prune

This command will prompt you for confirmation before proceeding. If you want to include unused images that are not currently referenced by any container as well, you can append the -a flag:

docker system prune -a

Considerations for Pruning

• Data Loss: Use this command with caution, as it will permanently delete unused data.
• Selective Pruning: If you're concerned about losing specific images, consider using the 

For instance, to remove only dangling images, you can run:

docker image prune

This will keep your actively used images intact while still cleaning your environment.

Summary

Managing Docker images is crucial for maintaining an efficient development and deployment workflow. Understanding how to identify and remove unused images not only helps save disk space but also ensures that your Docker environment runs smoothly.

By using commands like docker images, docker rmi, and docker system prune, you can effectively keep your workspace tidy. Remember that while these operations are straightforward, they carry the risk of data loss if not handled with care. Regular maintenance of Docker images will contribute to a more streamlined development process, allowing you to focus on building and deploying your applications rather than managing storage.

Last Update: 20 Jan, 2025