Squashing layers in Docker - Mini Project: Build & Apply

Squashing Layers in Docker Images

📖 Scenario: You are working on a Docker project where you want to optimize the image size by combining multiple layers into one. This helps reduce the final image size and speeds up deployment.

🎯 Goal: Build a Dockerfile that creates a base image with multiple commands, then squash the layers into a single layer using Docker build options.

📋 What You'll Learn

Create a Dockerfile with multiple RUN commands

Add a label to the image

Use Docker build with the --squash option to combine layers

Display the final image size

💡 Why This Matters

🌍 Real World

Squashing layers helps reduce Docker image size, which saves storage and speeds up deployment in real projects.

💼 Career

Understanding layer squashing is important for DevOps roles to optimize container images and improve CI/CD pipelines.

1

Create a Dockerfile with base image and multiple RUN commands

Create a file named Dockerfile with the following content exactly: start from the alpine:latest image, then add two RUN commands: one to update the package list with apk update and another to install curl with apk add curl.

Docker

# Your code here

Hint

Use FROM alpine:latest as the base. Then add two separate RUN lines for update and install.

2

Add a label to the Dockerfile

Add a LABEL instruction to the existing Dockerfile that sets maintainer to [e-mail].

Docker

FROM alpine:latest
RUN apk update
RUN apk add curl
# Add a LABEL instruction below

Hint

Use LABEL maintainer="[e-mail]" exactly.

3

Build the Docker image with squashing layers

Use the Docker CLI to build the image from the current directory with the tag myapp:squashed and enable layer squashing using the --squash option.

Docker

FROM alpine:latest
RUN apk update
RUN apk add curl
LABEL maintainer="[e-mail]"
# Your code here

Hint

Run docker build --squash -t myapp:squashed . in your terminal to build the image with squashed layers.

4

Display the final image size

Run the Docker command to show the size of the image tagged myapp:squashed.

Docker

FROM alpine:latest
RUN apk update
RUN apk add curl
LABEL maintainer="[e-mail]"
# Your code here

Hint

Use docker images myapp:squashed to see the image size.

Practice

1. What is the main purpose of docker build --squash?

easy

A. To speed up the build process by skipping some steps

B. To create multiple images from one Dockerfile

C. To combine multiple image layers into a single layer to reduce image size

D. To automatically update the base image to the latest version

Solution

1. Step 1: Understand what layers are in Docker images

   Docker images are made of layers created by each command in the Dockerfile.

2. Step 2: Purpose of squashing layers

   Squashing merges these layers into one to reduce the final image size and improve efficiency.

3. Final Answer:

   To combine multiple image layers into a single layer to reduce image size -> Option C

4. Quick Check:

   Squashing = combining layers [OK]

Hint: Squash means merge layers to save space [OK]

Common Mistakes:

• Thinking it speeds up build time
• Confusing squashing with creating multiple images
• Assuming it updates base images automatically

2. Which of the following is the correct syntax to build a Docker image with squashed layers?

easy

A. docker build -squash=true -t myimage .

B. docker build -squash -t myimage .

C. docker build --squash=true -t myimage .

D. docker build --squash -t myimage .

Solution

1. Step 1: Check the correct flag format for squashing

   The correct flag is --squash without an equals sign or single dash.

2. Step 2: Validate the full command syntax

   The command docker build --squash -t myimage . correctly uses the flag and tags the image.

3. Final Answer:

   docker build --squash -t myimage . -> Option D

4. Quick Check:

   Correct flag is double dash --squash [OK]

Hint: Use double dash --squash without equals sign [OK]

Common Mistakes:

• Using single dash -squash
• Adding =true after --squash
• Misplacing the flag after the context dot

3. Given this Dockerfile:

FROM alpine
RUN echo 'Hello' > /file1
RUN echo 'World' > /file2

What is the main difference in the image size when built with and without --squash?

medium

A. The image with --squash is smaller because layers are combined

B. The image with --squash is larger because it adds extra metadata

C. Both images have the same size because layers do not affect size

D. The image without --squash is smaller because layers are cached

Solution

1. Step 1: Understand how layers affect image size

   Each RUN command creates a new layer, adding to the total image size.

2. Step 2: Effect of --squash on layers

   Using --squash merges layers, removing intermediate data and reducing size.

3. Final Answer:

   The image with --squash is smaller because layers are combined -> Option A

4. Quick Check:

   Squash reduces size by merging layers [OK]

Hint: Squash merges layers, reducing image size [OK]

Common Mistakes:

• Thinking squashing adds metadata and increases size
• Believing layers do not affect image size
• Confusing caching with squashing effects

4. You run docker build --squash -t myapp . but get an error saying the flag is unknown. What is the most likely cause?

medium

A. The image tag is missing

B. Docker experimental features are not enabled

C. The Dockerfile has syntax errors

D. The build context path is incorrect

Solution

1. Step 1: Understand requirements for --squash flag

   The --squash flag requires Docker experimental features to be enabled.

2. Step 2: Identify cause of unknown flag error

   If experimental features are off, Docker does not recognize --squash and throws an error.

3. Final Answer:

   Docker experimental features are not enabled -> Option B

4. Quick Check:

   Enable experimental features to use --squash [OK]

Hint: Enable experimental features to use --squash [OK]

Common Mistakes:

• Assuming Dockerfile errors cause unknown flag
• Thinking missing tag causes flag error
• Blaming build context path for flag issues

5. You want to reduce your Docker image size by squashing layers but also keep some intermediate layers for caching. Which approach is best?

hard

A. Use multi-stage builds and squash only the final stage

B. Use --squash on every build to squash all layers

C. Avoid squashing and rely on layer caching only

D. Manually delete intermediate files after build

Solution

1. Step 1: Understand multi-stage builds

   Multi-stage builds allow creating intermediate images for caching and a final optimized image.

2. Step 2: Combine multi-stage builds with squashing

   Squashing only the final stage reduces image size while keeping intermediate layers for cache.

3. Final Answer:

   Use multi-stage builds and squash only the final stage -> Option A

4. Quick Check:

   Multi-stage + squash final stage = best size and cache [OK]

Hint: Squash final stage in multi-stage builds for best results [OK]

Common Mistakes:

• Squashing all layers loses caching benefits
• Avoiding squashing misses size reduction
• Deleting files manually is error-prone