Docker layer caching in CI - Mini Project: Build & Apply

Docker Layer Caching in CI

📖 Scenario: You are working on a project where you build Docker images frequently in a Continuous Integration (CI) pipeline. To save time and resources, you want to use Docker layer caching so that unchanged layers are reused instead of rebuilt every time.This project will guide you through setting up a simple Dockerfile and configuring a CI build script to use Docker layer caching effectively.

🎯 Goal: Build a Docker image with caching enabled in a CI environment to speed up repeated builds by reusing unchanged layers.

📋 What You'll Learn

Create a Dockerfile with multiple layers

Add a cache key variable in the CI build script

Use Docker build command with cache-from option

Print the Docker build output to verify caching

💡 Why This Matters

🌍 Real World

Docker layer caching helps speed up builds in CI pipelines by reusing unchanged layers, saving time and computing resources.

💼 Career

Understanding Docker caching is essential for DevOps engineers to optimize CI/CD pipelines and reduce build times in real projects.

1

Create a Dockerfile with multiple layers

Create a file named Dockerfile with these exact lines:
FROM python:3.12-slim
RUN apt-get update && apt-get install -y curl
COPY app.py /app/app.py
CMD ["python", "/app/app.py"]

Docker

# Create the Dockerfile with the specified content
# Your code here

Hint

Each line in the Dockerfile creates a layer. Make sure to write the lines exactly as shown.

2

Add a cache key variable in the CI build script

In a file named build.sh, create a variable called CACHE_KEY and set it to myapp-cache.

Docker

# Create CACHE_KEY variable
# Your code here

Hint

Just write CACHE_KEY=myapp-cache exactly in the script.

3

Use Docker build command with cache-from option

In the build.sh file, add a Docker build command that uses --cache-from with the image named $CACHE_KEY and tags the new image as myapp:latest.

Docker

# Add docker build command with --cache-from
# Your code here

Hint

Use the exact command docker build --cache-from=$CACHE_KEY -t myapp:latest .

4

Print the Docker build output to verify caching

Add a command in build.sh to print the output of the Docker build command so you can see if layers are cached.

Docker

# Print the output of the docker build command
# Your code here

Hint

Capture the output of the docker build command in a variable and then print it using echo.

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