What is Docker - Hands-On Activity

What is Docker

📖 Scenario: Imagine you want to share a small app with your friend. But your friend has a different computer setup. Docker helps you package your app so it runs the same everywhere.

🎯 Goal: Learn the basic idea of Docker by creating a simple Dockerfile that sets up a container with a message.

📋 What You'll Learn

Create a Dockerfile with a base image

Add a command to print a message

Build the Docker image

Run the Docker container to see the message

💡 Why This Matters

🌍 Real World

Docker is used to package apps so they run the same on any computer or server, avoiding setup problems.

💼 Career

Knowing Docker is essential for developers and DevOps engineers to build, share, and deploy applications efficiently.

1

Create a Dockerfile with a base image

Create a file named Dockerfile and write the line FROM alpine to use the Alpine Linux base image.

Docker

# Create a Dockerfile with the line: FROM alpine
# Your code here

Hint

Use the FROM instruction to specify the base image.

2

Add a command to print a message

Add the line CMD ["echo", "Hello from Docker!"] below the FROM alpine line in the Dockerfile.

Docker

FROM alpine
# Add a CMD instruction to print "Hello from Docker!"
# Your code here

Hint

Use CMD with JSON array syntax to specify the command.

3

Build the Docker image

Run the command docker build -t hello-docker . in the terminal to build the image named hello-docker from the current directory.

Docker

FROM alpine
CMD ["echo", "Hello from Docker!"]
# Build the Docker image with the tag hello-docker
# Your code here

Hint

Use docker build with -t to name your image.

4

Run the Docker container to see the message

Run the command docker run hello-docker in the terminal to start the container and see the message Hello from Docker!.

Docker

FROM alpine
CMD ["echo", "Hello from Docker!"]
# Command to build image:
docker build -t hello-docker .
# Run the container to see the message
# Your code here

Hint

Use docker run followed by the image name to start the container.

Practice

1. What is the main purpose of Docker?

easy

A. To write code faster using a new programming language

B. To package applications into containers that run consistently everywhere

C. To replace operating systems on servers

D. To create virtual machines for running multiple OS

Solution

1. Step 1: Understand Docker's core function

   Docker packages applications and their dependencies into containers.

2. Step 2: Compare with other options

   The other options describe unrelated technologies or incorrect uses.

3. Final Answer:

   To package applications into containers that run consistently everywhere -> Option B

4. Quick Check:

   Docker = container packaging [OK]

Hint: Docker = app in container for consistent runs [OK]

Common Mistakes:

• Confusing Docker with a programming language
• Thinking Docker replaces the OS
• Mixing Docker with virtual machines

2. Which of the following is the correct command to check the installed Docker version?

easy

A. docker version

B. docker check

C. docker --install

D. docker status

Solution

1. Step 1: Recall Docker commands

   The command to check Docker version is docker version.

2. Step 2: Validate other options

   Commands like docker check, docker --install, and docker status are invalid or do not show version.

3. Final Answer:

   docker version -> Option A

4. Quick Check:

   Check version = docker version [OK]

Hint: Use 'docker version' to see Docker's installed version [OK]

Common Mistakes:

• Using 'docker check' which is not a valid command
• Trying 'docker status' to check version
• Confusing install commands with version check

3. What will be the output of the command docker ps if no containers are running?

medium

A. Displays an empty table with headers but no container entries

B. Shows an error message 'No containers found'

C. Lists all running containers

D. Starts a new container automatically

Solution

1. Step 1: Understand 'docker ps' behavior

   The command docker ps lists running containers. If none are running, it shows headers but no container rows.

2. Step 2: Check other options

   It does not show an error or start containers automatically.

3. Final Answer:

   Displays an empty table with headers but no container entries -> Option A

4. Quick Check:

   docker ps empty = headers only [OK]

Hint: 'docker ps' shows running containers or empty headers if none [OK]

Common Mistakes:

• Expecting an error message when no containers run
• Thinking it starts containers automatically
• Confusing 'docker ps' with 'docker images'

4. You run docker run hello-world but get an error saying the image is not found locally. What should you do to fix this?

medium

A. Delete all containers and try again

B. Reinstall Docker completely

C. Run docker pull hello-world to download the image first

D. Change the command to docker start hello-world

Solution

1. Step 1: Understand the error cause

   The error means the image 'hello-world' is not on your machine.

2. Step 2: Fix by downloading the image

   Use docker pull hello-world to get the image from Docker Hub.

3. Final Answer:

   Run docker pull hello-world to download the image first -> Option C

4. Quick Check:

   Image missing? Use docker pull [OK]

Hint: Pull missing images before running containers [OK]

Common Mistakes:

• Trying to start a container without the image
• Reinstalling Docker unnecessarily
• Deleting containers unrelated to the image issue

5. Which of the following best explains why Docker containers help teams work faster and more reliably?

hard

A. Containers automatically fix bugs in the application code

B. Containers run only on the developer's local machine

C. Containers replace the need for any testing before deployment

D. Containers isolate apps and dependencies, avoiding setup conflicts across environments

Solution

1. Step 1: Understand container benefits

   Docker containers package apps with all needed parts, so they run the same everywhere.

2. Step 2: Evaluate options

   Containers isolate apps and dependencies, avoiding setup conflicts across environments correctly states isolation and consistency. The other options are false or misleading.

3. Final Answer:

   Containers isolate apps and dependencies, avoiding setup conflicts across environments -> Option D

4. Quick Check:

   Isolation + consistency = faster, reliable work [OK]

Hint: Containers isolate apps to avoid environment issues [OK]

Common Mistakes:

• Believing containers fix code bugs automatically
• Skipping testing because of containers
• Thinking containers only run locally