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.

Practice

1. What is the basic Docker command to remove an image from your system?

easy

A. docker delete <image_name_or_id>

B. docker rmi <image_name_or_id>

C. docker remove image <image_name_or_id>

D. docker rm <image_name_or_id>

Solution

1. Step 1: Understand Docker image removal command

   The correct command to remove Docker images is docker rmi. It stands for 'remove image'.

2. Step 2: Identify the correct syntax

   The command requires the image name or ID after docker rmi to specify which image to remove.

3. Final Answer:

   docker rmi <image_name_or_id> -> Option B

4. Quick Check:

   Remove image = docker rmi [OK]

Hint: Use 'docker rmi' to remove images quickly [OK]

Common Mistakes:

• Using 'docker rm' which removes containers, not images
• Typing 'docker delete' which is not a valid Docker command
• Confusing image removal with container removal commands

2. Which of the following is the correct syntax to force remove a Docker image named myapp:latest?

easy

A. docker rmi -f myapp:latest

B. docker rmi -force myapp:latest

C. docker rmi -rf myapp:latest

D. docker rmi --f myapp:latest

Solution

1. Step 1: Identify the correct flag for force removal

   The short flag -f is used to force remove images in Docker.

2. Step 2: Confirm the correct command syntax

   The correct command is docker rmi -f myapp:latest. Other options use invalid flags like -rf, -force, or --f.

3. Final Answer:

   docker rmi -f myapp:latest -> Option A

4. Quick Check:

   Force remove = docker rmi -f [OK]

Hint: Use '-f' to force remove images safely [OK]

Common Mistakes:

• Using incorrect flag like '-rf', '-force' or '--f'
• Confusing force flag with other options
• Misspelling the flag causing command failure

3. What will be the output of the command docker rmi 123abc456def if the image ID 123abc456def exists and is not used by any container?

medium

A. Image removed but container still running

B. Error: image is being used by a running container

C. No such image: 123abc456def

D. Untagged: 123abc456def
Deleted: sha256:123abc456def...

Solution

1. Step 1: Check if image exists and is unused

   If the image ID exists and no container uses it, Docker removes it cleanly.

2. Step 2: Understand Docker rmi output

   The command outputs lines showing the image is untagged and deleted with its sha256 hash.

3. Final Answer:

   Untagged: 123abc456def Deleted: sha256:123abc456def... -> Option D

4. Quick Check:

   Image removed cleanly = Untagged: 123abc456def Deleted: sha256:123abc456def... [OK]

Hint: If image unused, 'docker rmi' shows untagged and deleted [OK]

Common Mistakes:

• Assuming image removal works if container uses it
• Expecting no output on successful removal
• Confusing image ID with container ID

4. You run docker rmi myimage but get the error: Error response from daemon: conflict: unable to delete image (must be forced) - image is being used by running container. What should you do to fix this?

medium

A. Use docker rm myimage to remove the image

B. Run docker rmi myimage again without changes

C. Stop the running container using the image, then run docker rmi myimage again

D. Restart Docker daemon and try again

Solution

1. Step 1: Understand the error meaning

   The error says the image is used by a running container, so Docker won't remove it without force.

2. Step 2: Stop the container using the image

   You must stop or remove the container using the image before removing the image itself.

3. Final Answer:

   Stop the running container using the image, then run docker rmi myimage again -> Option C

4. Quick Check:

   Stop container before removing image [OK]

Hint: Stop containers using image before removal [OK]

Common Mistakes:

• Trying to remove image without stopping containers
• Using 'docker rm' which removes containers, not images
• Restarting Docker daemon unnecessarily

5. You want to remove all Docker images that are not used by any container to free up space. Which command will safely do this?

hard

A. docker image prune -a

B. docker rmi $(docker images -q)

C. docker rmi -f $(docker ps -aq)

D. docker container prune

Solution

1. Step 1: Understand the goal

   You want to remove all unused images, but keep images used by containers.

2. Step 2: Identify the correct command

   docker image prune -a removes all unused images safely without affecting running containers.

3. Step 3: Analyze other options

   docker rmi $(docker images -q) tries to remove all images, including used ones, causing errors.
   docker rmi -f $(docker ps -aq) is invalid because docker ps -aq lists containers, not images.
   docker container prune removes stopped containers, not images.

4. Final Answer:

   docker image prune -a -> Option A

5. Quick Check:

   Remove unused images safely = docker image prune -a [OK]

Hint: Use 'docker image prune -a' to clean unused images [OK]

Common Mistakes:

• Trying to remove all images forcibly causing errors
• Confusing container prune with image prune
• Using container IDs instead of image IDs in commands