Why Dockerfiles automate image creation - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the Dockerfile command to specify the base image.

Docker

FROM [1]

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

Aubuntu:latest

BRUN apt-get update

CCOPY . /app

DCMD ["bash"]

Attempts:

3 left

💡 Hint

Common Mistakes

Using RUN or COPY instead of FROM to specify the base image.

The FROM command sets the base image for the Docker image. Here, ubuntu:latest is a common base.

2fill in blank

medium

Complete the Dockerfile command to copy files into the image.

Docker

COPY [1] /app

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

ACMD ["python"]

BRUN apt-get install

C/app

D.

Attempts:

3 left

💡 Hint

Common Mistakes

Using commands like RUN or CMD instead of a path for COPY.

The COPY command copies files from the current directory . into the image's /app folder.

3fill in blank

hard

Fix the error in the Dockerfile command to run a shell command.

Docker

[1] apt-get update

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

ARUN

BCMD

CCOPY

Dapt-get

Attempts:

3 left

💡 Hint

Common Mistakes

Using COPY or CMD instead of RUN for shell commands.

The RUN command executes shell commands during image build. Here, it runs apt-get update.

4fill in blank

hard

Fill both blanks to set the working directory and default command.

Docker

WORKDIR [1]
CMD [2]

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

A/app

B["python", "app.py"]

C["bash"]

D/usr/src

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up WORKDIR and CMD values.

WORKDIR sets the folder where commands run. CMD sets the default command to run when the container starts.

5fill in blank

hard

Fill all three blanks to create a dictionary of environment variables in a Dockerfile.

Docker

ENV [1]=[2] [3]

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

AAPP_ENV

Bproduction

CDEBUG=false

DPATH

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid syntax or missing equal signs in ENV.

The ENV command sets environment variables inside the image. Here, APP_ENV=production and DEBUG=false are set.

Practice

1. Why do Dockerfiles help automate the creation of Docker images?

easy

A. They replace the need for Docker Engine.

B. They run containers faster than manual commands.

C. They list all the steps needed to build the image automatically.

D. They store data inside running containers.

Solution

1. Step 1: Understand Dockerfile purpose

   A Dockerfile contains instructions that tell Docker how to build an image step-by-step.

2. Step 2: Recognize automation benefit

   By listing steps, Dockerfiles automate image creation, avoiding manual commands each time.

3. Final Answer:

   They list all the steps needed to build the image automatically. -> Option C

4. Quick Check:

   Dockerfile = step list for image build [OK]

Hint: Dockerfiles are scripts listing build steps [OK]

Common Mistakes:

• Thinking Dockerfiles run containers
• Confusing Dockerfiles with Docker Engine
• Believing Dockerfiles store container data

2. Which of the following is the correct syntax to start a Dockerfile?

easy

A. FROM ubuntu:latest

B. RUN ubuntu:latest

C. BUILD ubuntu:latest

D. START ubuntu:latest

Solution

1. Step 1: Identify Dockerfile base image instruction

   The Dockerfile must start with FROM to specify the base image.

2. Step 2: Check syntax correctness

   Only 'FROM ubuntu:latest' is valid syntax to start a Dockerfile.

3. Final Answer:

   FROM ubuntu:latest -> Option A

4. Quick Check:

   FROM starts Dockerfile [OK]

Hint: Dockerfiles always start with FROM [OK]

Common Mistakes:

• Using RUN or BUILD as first line
• Confusing command names
• Omitting the FROM instruction

3. Given this Dockerfile snippet:

FROM alpine
RUN apk add --no-cache curl
CMD ["curl", "--version"]

What happens when you build and run this image?

medium

A. The container installs curl but does not run it.

B. The container prints the curl version and exits.

C. The build fails because CMD is incorrect.

D. The container runs but shows no output.

Solution

1. Step 1: Analyze Dockerfile instructions

   The image starts from alpine, installs curl, then sets CMD to run 'curl --version'.

2. Step 2: Understand container run behavior

   When run, the container executes CMD, printing curl version, then exits.

3. Final Answer:

   The container prints the curl version and exits. -> Option B

4. Quick Check:

   CMD runs curl --version on container start [OK]

Hint: CMD runs command when container starts [OK]

Common Mistakes:

• Thinking CMD runs during build
• Assuming no output is shown
• Believing build fails due to CMD

4. You wrote this Dockerfile:

FROM node:18
RUN npm install express
CMD ["node", "app.js"]

But when you run the container, it fails. What is the likely problem?

medium

A. You forgot to copy your app.js file into the image.

B. The FROM instruction is invalid for Node.js images.

C. npm install express should be CMD, not RUN.

D. CMD syntax is incorrect; it needs shell form.

Solution

1. Step 1: Check Dockerfile steps

   It installs express but never copies app.js into the image.

2. Step 2: Understand runtime failure cause

   Without app.js, running 'node app.js' fails because file is missing.

3. Final Answer:

   You forgot to copy your app.js file into the image. -> Option A

4. Quick Check:

   Missing app.js causes runtime failure [OK]

Hint: Always COPY app files before running them [OK]

Common Mistakes:

• Thinking FROM node:18 is invalid
• Confusing RUN and CMD usage
• Believing CMD needs shell form

5. How does using a Dockerfile improve team collaboration when creating Docker images?

hard

A. It automatically updates images without team input.

B. It stores all container logs for team review.

C. It allows running containers without Docker Engine installed.

D. It provides a clear, repeatable script to build the same image everywhere.

Solution

1. Step 1: Understand Dockerfile role in collaboration

   Dockerfiles list exact build steps, so anyone can build the same image reliably.

2. Step 2: Recognize benefits for teams

   This repeatability avoids differences and errors, making sharing and reproducing environments easy.

3. Final Answer:

   It provides a clear, repeatable script to build the same image everywhere. -> Option D

4. Quick Check:

   Dockerfile = shared build script for teams [OK]

Hint: Dockerfiles ensure consistent builds for all team members [OK]

Common Mistakes:

• Thinking Dockerfiles auto-update images
• Believing Dockerfiles replace Docker Engine
• Confusing Dockerfiles with logging tools