Removing images in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of removing a non-existent Docker image

What is the output when you run docker rmi non_existent_image on a system where this image does not exist?

Docker

docker rmi non_existent_image

A Untagged: non_existent_image:latest

B Image removed successfully

C Deleted: sha256:abcdef1234567890

D Error response from daemon: No such image: non_existent_image

💡 Hint

Think about what Docker does if the image name is not found locally.

Docker will return an error message indicating that the image does not exist locally when you try to remove a non-existent image.

💻 Command Output

intermediate

Effect of force removing a Docker image

What happens when you run docker rmi -f image_name if the image is currently used by a running container?

Docker

docker rmi -f image_name

A The image is removed but the running container continues to run

B The command hangs until the container is stopped manually

C Error: conflict: unable to remove image because it is being used by a running container

D The image is removed and the running container stops immediately

💡 Hint

Consider what the force flag does in Docker image removal.

Using the force flag removes the image even if containers are using it, but running containers continue to run using the image layers already loaded.

❓ Configuration

advanced

Docker command to remove all dangling images

Which command correctly removes all dangling (unused) Docker images?

A docker rmi $(docker images -f "dangling=true" -q)

B docker rmi $(docker ps -a -q)

C docker rmi --all

D docker rmi -a dangling

💡 Hint

Dangling images are those without tags and not used by any container.

The command uses docker images filter to list dangling images and removes them by their IDs.

❓ Troubleshoot

advanced

Reason for failure when removing an image with dependent containers

You try to remove an image with docker rmi image_name but get the error: conflict: unable to remove repository reference "image_name" (must force) - container 123abc is using its referenced image. What is the cause?

A The image name is misspelled and does not exist

B There is a running or stopped container using the image, preventing removal without force

C Docker daemon is not running, so the command fails

D The image is corrupted and cannot be removed

💡 Hint

Think about what Docker protects when containers depend on images.

Docker prevents removing images that are referenced by containers unless the force flag is used.

✅ Best Practice

expert

Best practice to safely remove unused Docker images in a CI/CD pipeline

In a CI/CD pipeline, which approach is safest to remove unused Docker images without affecting running containers or builds?

A Manually remove images by name after stopping all containers

B Run <code>docker rmi -f $(docker images -q)</code> to force remove all images

C Run <code>docker image prune -a --filter "until=24h"</code> to remove images unused for over 24 hours

D Delete the Docker image storage directory on the host filesystem

💡 Hint

Consider automated cleanup that avoids disrupting running containers.

Using docker image prune with filters safely removes unused images older than a set time, avoiding impact on running containers.