Image layers concept in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to build a Docker image from a Dockerfile.

Docker

docker build -t myapp [1] Dockerfile .

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

A-d

B-f

C--rm

D--push

Attempts:

3 left

💡 Hint

Common Mistakes

Using -d which is for detached mode in run command

Using --push which is for pushing images

Using --rm which removes intermediate containers

The -f option specifies the Dockerfile to use when building the image.

2fill in blank

medium

Complete the Dockerfile line to add a new file to the image.

Docker

COPY [1] /app/

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

Afile.txt

B./app/file.txt

C/app/file.txt

D/file.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Using absolute paths which are not in the build context

Adding destination path in the source position

The COPY command copies a file from the build context (local folder) to the image. The source is just the filename.

3fill in blank

hard

Fix the error in the Dockerfile line that sets the working directory.

Docker

WORKDIR [1]app

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

A/

B./

C/home/

Attempts:

3 left

💡 Hint

Common Mistakes

Using relative paths like 'app' or './app'

Missing the leading slash for absolute path

The WORKDIR path should be an absolute path starting with a slash. Using /app sets the working directory to /app.

4fill in blank

hard

Fill both blanks to create a Dockerfile line that runs a command and creates a new image layer.

Docker

RUN [1] && [2]

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

Aapt-get update

Bapt-get install -y curl

Cecho 'Hello World'

Drm -rf /var/lib/apt/lists/*

Attempts:

3 left

💡 Hint

Common Mistakes

Running install before update

Using echo which does not install software

The RUN command executes shell commands and creates a new image layer. Updating package lists and installing curl are common steps.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps image layers to their sizes if size is greater than zero.

Docker

layer_sizes = { [1]: [2] for [3], size in layers.items() if size > 0 }

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

Alayer

Bsize

Dlayer.upper()

Attempts:

3 left

💡 Hint

Common Mistakes

Using layer instead of layer.upper() for keys

Mixing up keys and values in comprehension

This comprehension creates a dictionary with layer names in uppercase as keys and their sizes as values, filtering out layers with size zero.

Practice

1. What does each layer in a Docker image represent?

easy

A. A user account inside the container

B. A separate container running inside Docker

C. A network connection between containers

D. A change made by a single Dockerfile command

Solution

1. Step 1: Understand Dockerfile commands

   Each command in a Dockerfile creates a change in the image.

2. Step 2: Connect changes to layers

   Docker saves each change as a separate layer to reuse if unchanged.

3. Final Answer:

   A change made by a single Dockerfile command -> Option D

4. Quick Check:

   Layer = Dockerfile command change [OK]

Hint: Think of layers as snapshots after each Dockerfile step [OK]

Common Mistakes:

• Confusing layers with running containers
• Thinking layers are user accounts
• Believing layers are network connections

2. Which Dockerfile command creates a new image layer?

easy

A. START

B. ENVIRONMENT

C. RUN

D. COPYFILE

Solution

1. Step 1: Identify valid Dockerfile commands

   RUN is a valid Dockerfile command to execute commands in the image.

2. Step 2: Check other options

   ENVIRONMENT, COPYFILE, and START are not valid Dockerfile commands.

3. Final Answer:

   RUN -> Option C

4. Quick Check:

   RUN creates layers [OK]

Hint: Only standard Dockerfile commands create layers [OK]

Common Mistakes:

• Using incorrect or non-existent Dockerfile commands
• Confusing ENVIRONMENT with ENV
• Thinking COPYFILE is a Docker command

3. Given this Dockerfile snippet:

FROM ubuntu
RUN apt-get update
RUN apt-get install -y curl
COPY . /app

How many layers will this image have (excluding the base image)?

medium

A. 3

B. 2

C. 1

D. 4

Solution

1. Step 1: Count commands creating layers

   RUN apt-get update, RUN apt-get install -y curl, and COPY . /app each create a layer.

2. Step 2: Exclude base image layer

   FROM ubuntu is the base image, not counted here.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   3 commands create layers [OK]

Hint: Count RUN, COPY, ADD commands for layers [OK]

Common Mistakes:

• Counting FROM as a new layer
• Ignoring COPY command creates a layer
• Miscounting number of RUN commands

4. You notice your Docker image size is very large. Which Dockerfile practice can help reduce image layers and size?

medium

A. Combine multiple RUN commands into one using &&

B. Add a new RUN command for each small step

C. Use multiple COPY commands for each file

D. Use FROM multiple times in one Dockerfile

Solution

1. Step 1: Understand layer creation

   Each RUN command creates a new layer, increasing image size.

2. Step 2: Combine commands to reduce layers

   Using && to combine commands in one RUN reduces layers and image size.

3. Final Answer:

   Combine multiple RUN commands into one using && -> Option A

4. Quick Check:

   Fewer RUN commands = fewer layers [OK]

Hint: Merge RUN commands with && to shrink layers [OK]

Common Mistakes:

• Adding many small RUN commands
• Using multiple COPY commands unnecessarily
• Misusing multiple FROM commands in one Dockerfile

5. You want to optimize a Docker image build where a large file changes frequently. Which approach best minimizes rebuild time and image size?

hard

A. Place COPY of the large file at the start of the Dockerfile

B. Place COPY of the large file at the end of the Dockerfile

C. Combine COPY of the large file with RUN commands in one layer

D. Avoid using COPY and download the file inside RUN

Solution

1. Step 1: Understand layer caching

   Docker caches layers; if a layer changes, later layers rebuild.

2. Step 2: Place frequently changing files last

   Putting COPY of large changing files last limits rebuild to that layer only.

3. Final Answer:

   Place COPY of the large file at the end of the Dockerfile -> Option B

4. Quick Check:

   Last COPY limits rebuild scope [OK]

Hint: Put changing files COPY last to speed rebuilds [OK]

Common Mistakes:

• Copying large files early causing full rebuilds
• Combining COPY with RUN causing bigger layers
• Downloading files inside RUN increasing image size