Docker layer caching in CI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Docker layer caching in CI?

Docker layer caching in CI means saving parts of the Docker image build steps so that future builds can reuse them. This makes builds faster by not repeating unchanged steps.

beginner

How does Docker layer caching speed up builds?

Docker breaks images into layers for each build step. If a layer hasn’t changed, Docker reuses it instead of rebuilding. This saves time and computing resources.

intermediate

Why is Docker layer caching challenging in CI environments?

CI systems often start fresh environments for each build, so cached layers are lost unless explicitly saved and restored between builds.

intermediate

Name a common method to enable Docker layer caching in CI pipelines.

One common method is to push built image layers to a remote Docker registry after a build, then pull them before the next build to reuse cached layers.

intermediate

What is the role of the Dockerfile order in layer caching?

The order of commands in a Dockerfile affects caching. Changes early in the file invalidate all following layers, so placing stable commands first helps maximize cache reuse.

What happens if a Docker build step changes in a CI pipeline with caching enabled?

A The build fails

B All steps are rebuilt from the start

C No steps are rebuilt

D Only the changed step and following steps are rebuilt

Docker rebuilds the changed step and all steps after it, but reuses cached layers before the change.

Why might Docker layer caching not work automatically in CI systems?

A CI systems delete all files after each build

B Docker does not support caching

C CI environments are often clean and do not keep previous cache layers unless saved

D Dockerfiles are incompatible with CI

CI environments usually start fresh, so cached layers must be saved and restored explicitly.

Which of these helps improve Docker layer caching efficiency?

A Randomizing Dockerfile commands

B Placing frequently changing commands at the end of the Dockerfile

C Using only one RUN command

D Avoiding any caching

Placing changing commands at the end keeps earlier layers stable and cached.

What is a common way to share Docker cache between CI builds?

A Using a shared Docker registry to push and pull images

B Rebuilding everything every time

C Disabling Docker caching

D Using a different Dockerfile each time

Pushing and pulling images from a registry allows reuse of cached layers across builds.

Which Docker command helps to build an image using cache from a remote registry?

A docker build --cache-from

B docker run --cache

C docker pull --cache

D docker push --cache

The --cache-from option tells Docker to use layers from a specified image as cache.

Explain how Docker layer caching works and why it is useful in CI pipelines.

Think about how Docker saves time by not repeating work.

Describe strategies to enable Docker layer caching in a CI environment.

Consider how to keep cache available between fresh CI runs.

Practice

1. What is the main purpose of Docker layer caching in Continuous Integration (CI)?

easy

A. To speed up Docker image builds by reusing unchanged layers

B. To increase the size of Docker images

C. To delete unused Docker images automatically

D. To run Docker containers faster

Solution

1. Step 1: Understand Docker layers

   Docker images are built in layers, each representing a step in the Dockerfile.

2. Step 2: Purpose of caching layers in CI

   Reusing unchanged layers avoids rebuilding them, which speeds up the build process in CI pipelines.

3. Final Answer:

   To speed up Docker image builds by reusing unchanged layers -> Option A

4. Quick Check:

   Docker layer caching = speed up builds [OK]

Hint: Caching reuses unchanged steps to build faster [OK]

Common Mistakes:

• Thinking caching deletes images
• Confusing caching with container runtime speed
• Assuming caching increases image size

2. Which Docker build command option is used to specify a cache source in a CI pipeline?

easy

A. --rm

B. --build-arg

C. --no-cache

D. --cache-from

Solution

1. Step 1: Identify cache-related options

   Docker build supports options like --cache-from to specify images to use as cache sources.

2. Step 2: Match option to cache source usage

   --cache-from tells Docker to use layers from the specified image as cache, which is essential in CI for caching.

3. Final Answer:

   --cache-from -> Option D

4. Quick Check:

   Cache source option = --cache-from [OK]

Hint: Use --cache-from to specify cache images in CI builds [OK]

Common Mistakes:

• Using --no-cache disables caching
• Confusing --build-arg with caching
• Thinking --rm affects caching

3. Given this Docker build command in a CI pipeline:

docker build --cache-from myrepo/myimage:latest -t myrepo/myimage:latest .

What happens if the base image layers have not changed since the last build?

medium

A. Docker rebuilds all layers from scratch

B. Docker uses cached layers from myrepo/myimage:latest to speed up the build

C. Docker ignores the cache and pulls a new base image

D. Docker fails with an error about missing cache

Solution

1. Step 1: Understand --cache-from behavior

   The --cache-from option allows Docker to reuse layers from the specified image if they match the current build steps.

2. Step 2: Effect when base image layers are unchanged

   If the base image layers are unchanged, Docker will reuse those cached layers, speeding up the build process.

3. Final Answer:

   Docker uses cached layers from myrepo/myimage:latest to speed up the build -> Option B

4. Quick Check:

   Unchanged layers reused = faster build [OK]

Hint: Unchanged layers reuse cache with --cache-from [OK]

Common Mistakes:

• Assuming Docker always rebuilds all layers
• Thinking cache-from pulls new images
• Expecting errors if cache is missing

4. A CI pipeline uses Docker layer caching with --cache-from, but caching is not working and builds are slow. What is the most likely cause?

medium

A. The cache image is not pushed to or pulled from the registry

B. The base image is too large

C. The Dockerfile has syntax errors

D. The Docker daemon is not running

Solution

1. Step 1: Check cache image availability

   For caching to work in CI, the cache image must be pushed to a registry and pulled before building.

2. Step 2: Identify common caching failure cause

   If the cache image is missing or not pulled, Docker cannot reuse layers, causing slow builds.

3. Final Answer:

   The cache image is not pushed to or pulled from the registry -> Option A

4. Quick Check:

   Cache image missing = no caching [OK]

Hint: Ensure cache image is pushed and pulled in CI [OK]

Common Mistakes:

• Blaming Dockerfile syntax for caching issues
• Ignoring cache image push/pull steps
• Assuming base image size affects caching directly

5. In a CI pipeline, you want to maximize Docker layer caching efficiency across multiple builds. Which approach is best?

hard

A. Use --no-cache to force fresh builds every time

B. Build images locally on the CI server without pushing

C. Push the built image to a registry and use --cache-from with that image in subsequent builds

D. Delete all images after each build to save space

Solution

1. Step 1: Understand persistent caching in CI

   To reuse cache between builds, the cache must be stored persistently, usually by pushing images to a registry.

2. Step 2: Use cache-from with pushed images

   Using --cache-from with the pushed image allows Docker to reuse layers from previous builds, speeding up future builds.

3. Final Answer:

   Push the built image to a registry and use --cache-from with that image in subsequent builds -> Option C

4. Quick Check:

   Persistent cache storage + --cache-from = efficient caching [OK]

Hint: Push images and use --cache-from for persistent caching [OK]

Common Mistakes:

• Using --no-cache disables caching
• Deleting images removes cache
• Building locally without pushing loses cache between runs