Image layers concept in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Docker image layers

Which statement best describes Docker image layers?

A Layers are stacked read-only filesystems that build the final image.

B Layers are temporary and deleted after container creation.

C Each layer is a full copy of the entire image filesystem.

D Layers are only used for container runtime, not image building.

💡 Hint

Think about how Docker reuses parts of images to save space.

Docker images are made of stacked read-only layers. Each layer adds or modifies files. This stacking allows efficient storage and sharing.

💻 Command Output

intermediate

Inspecting image layers with Docker CLI

What is the output of the command docker history alpine?

Docker

docker history alpine

A A list showing image layers with size, creation time, and commands used.

B A list of running containers based on alpine image.

C The full Dockerfile used to build alpine image.

D An error stating 'alpine' image not found.

💡 Hint

Try to recall what docker history shows about images.

docker history lists the layers of an image, showing size, creation time, and commands that created each layer.

🔀 Workflow

advanced

Optimizing Dockerfile to reduce image layers

You want to reduce the number of layers in your Docker image. Which Dockerfile snippet achieves this best?

Docker

RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

A RUN apt-get update; apt-get install -y curl; rm -rf /var/lib/apt/lists/*

B

RUN apt-get update
RUN apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

C RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

D

RUN apt-get update && apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

💡 Hint

Think about how Docker creates a new layer for each RUN instruction.

Combining commands with && in a single RUN instruction creates only one layer, reducing image size.

❓ Troubleshoot

advanced

Diagnosing unexpected large Docker image size

Your Docker image is unexpectedly large. Which cause is most likely related to image layers?

A You used COPY instead of ADD for files.

B You used ENV variables in the Dockerfile.

C You did not specify a base image in the Dockerfile.

D You used multiple RUN commands that add temporary files without cleanup.

💡 Hint

Think about how layers keep all changes, even deleted files if not cleaned properly.

Each RUN creates a layer. If temporary files are created and not removed in the same RUN, they remain in the layer, increasing image size.

✅ Best Practice

expert

Choosing the best practice for layer caching in Docker builds

Which Dockerfile practice best leverages layer caching to speed up rebuilds?

A Place frequently changing commands at the top of the Dockerfile.

B Order commands from least to most frequently changing to maximize cache reuse.

C Combine all commands into one RUN instruction regardless of change frequency.

D Avoid using COPY or ADD commands to prevent cache invalidation.

💡 Hint

Think about how Docker caches layers and when cache is invalidated.

Docker caches layers in order. If a layer changes, all following layers are rebuilt. Ordering commands from least to most frequently changing maximizes cache reuse.