Squashing layers in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile command to squash layers during build.

Docker

docker build [1] .

Drag options to blanks, or click blank then click option'

A--tag

B--rm

C--squash

D--pull

Attempts:

3 left

💡 Hint

Common Mistakes

Using --tag instead of --squash

Forgetting to add the --squash option

Using --rm which removes intermediate containers but does not squash layers

The --squash option tells Docker to squash all layers into one during the build.

2fill in blank

medium

Complete the Docker build command to tag the image while squashing layers.

Docker

docker build [1] myimage:latest --squash .

Drag options to blanks, or click blank then click option'

A--tag

B--rm

C--quiet

D--pull

Attempts:

3 left

💡 Hint

Common Mistakes

Using --pull which updates base images but does not tag

Using --rm which removes intermediate containers

Using --quiet which suppresses output

The --tag option assigns a name and tag to the image being built.

3fill in blank

hard

Fix the error in this Dockerfile line to squash layers by combining RUN commands.

Docker

RUN apt-get update && apt-get install -y curl && [1]

Drag options to blanks, or click blank then click option'

Aapt-get clean

Bapt-get remove

Capt-get upgrade

Dapt-get autoremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using apt-get remove which deletes packages

Using apt-get upgrade which updates packages but does not clean

Using apt-get autoremove which removes unused packages but not cache

Adding apt-get clean after installing packages helps reduce image size by cleaning cache, effectively squashing changes in one layer.

4fill in blank

hard

Fill both blanks to create a squashed image by combining commands in one RUN statement.

Docker

RUN [1] && [2] && rm -rf /var/lib/apt/lists/*

Drag options to blanks, or click blank then click option'

Aapt-get update

Bapt-get install -y git

Capt-get clean

Dapt-get upgrade

Attempts:

3 left

💡 Hint

Common Mistakes

Using apt-get clean before install

Using apt-get upgrade instead of install

Not chaining commands properly

Combining apt-get update and apt-get install -y git in one RUN command reduces layers and helps squash them.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that filters and transforms Docker image tags.

Docker

tags = {tag[1]:tag[2] for tag in images if tag[3] 'latest'}

Drag options to blanks, or click blank then click option'

A.split('-')[0]

B.upper()

C!=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!=' in the filter

Not splitting the tag for the key

Not converting tag to uppercase

This comprehension splits the tag at '-', takes the first part as key, converts the tag to uppercase as value, and filters out tags equal to 'latest'.

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