Docker Remove Unused Images

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In this guide, we will see how Docker's commands help you maintain a lean and efficient Docker ecosystem by removing unused images.

Unused images are images that do not have a running or stopped container associated with them.

Check out all Docker tutorials and guides: Docker Tutorials and Guides

The Cost of Unused Docker Images

Before diving into the cleanup, it's essential to understand the impact of neglected images: 
Storage Overhead: Each image represents a snapshot of a containerized application or service. Over time, these can accumulate and take up a considerable amount of disk space. 

Cluttered Environment: Having numerous unnecessary images can make it difficult to identify the ones you actually need, leading to a cluttered and confusing environment. 

Potential Security Risks: Old and unused images might contain vulnerabilities. Regularly pruning your images helps reduce your exposure to potential security threats.

Docker’s Garbage Collection - docker system prune 

Docker provides a handy command to help clean up resources:
docker system prune
By default, this command will: 
  • Remove all stopped containers. 
  • Delete all networks not used by at least one container. 
  • Remove all dangling images (those without a tag). 
  • Remove all build cache. 
If you specifically want to remove unused images, including those that are tagged and not attached to any container:
docker system prune -a
The complete output:
docker system prune -a
WARNING! This will remove:
  - all stopped containers
  - all networks not used by at least one container
  - all images without at least one container associated to them
  - all build cache

Are you sure you want to continue? [y/N]

Docker Image Prune

If we do not want to find dangling images and remove them one by one, we can use the docker image prune command. This command removes all dangling images. If we also want to remove unused images, we can use the -a flag. 

Let's run the below command:
docker image prune -a
WARNING! This will remove all images without at least one container associated to them.
Are you sure you want to continue? [y/N] y
The command will return the list of image IDs that were removed and the space that was freed.

Targeting Images Directly

If you want more granular control and wish to target images specifically, Docker offers commands for that as well.

List All Images:

docker images -a

Remove a Specific Image: 

Find the IMAGE ID from the list and:

docker rmi <IMAGE_ID>

Best Practices for Image Management

Regular Cleanup: Schedule regular cleanup activities. This could be a manual process or automated using cron jobs or similar task schedulers. 

Tag Management: When building images, use meaningful tags to help identify the image's purpose and lifecycle. 

Monitor Disk Usage: Keep an eye on the storage used by Docker. Tools like docker system df can provide a quick overview of disk usage by Docker objects. 

Conclusion

Regularly purging unused Docker images ensures that your system remains optimized and free from unnecessary clutter. While Docker provides robust tools to manage images, users must wield them judiciously, ensuring valuable data isn’t unintentionally discarded. Happy Dockerizing!

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