Building images with docker build - Mini Project: Build & Apply

Building images with docker build

📖 Scenario: You are working as a DevOps engineer. Your team needs a Docker image for a simple web application. You will create a Dockerfile, set a base image, add a configuration, build the image, and then check the image is created.

🎯 Goal: Build a Docker image step-by-step using the docker build command and a Dockerfile. You will create the Dockerfile, add a base image, add a label, build the image with a tag, and finally list the images to confirm the build.

📋 What You'll Learn

Create a Dockerfile with the exact name Dockerfile

Use the base image alpine:3.18

Add a label maintainer="[e-mail]"

Build the Docker image with the tag mywebapp:1.0

List Docker images to confirm the new image exists

💡 Why This Matters

🌍 Real World

Building Docker images is a key step in packaging applications for deployment in containers. This project simulates creating a basic image for a web app.

💼 Career

DevOps engineers and developers use Docker images to create consistent environments. Knowing how to build and tag images is essential for container workflows.

1

Create a Dockerfile with a base image

Create a file named Dockerfile and write the line FROM alpine:3.18 to set the base image.

Docker

# Create a Dockerfile with the line 'FROM alpine:3.18'
# Your code here

Hint

The first line in a Dockerfile usually starts with FROM followed by the base image name and tag.

2

Add a maintainer label to the Dockerfile

Add the line LABEL maintainer="[e-mail]" below the FROM alpine:3.18 line in the Dockerfile.

Docker

FROM alpine:3.18
# Add a LABEL line with maintainer="[e-mail]" below
# Your code here

Hint

The LABEL instruction adds metadata to the image. Use the exact key and value as shown.

3

Build the Docker image with a tag

Run the command docker build -t mywebapp:1.0 . in the terminal to build the Docker image using the current directory's Dockerfile and tag it as mywebapp:1.0.

Docker

FROM alpine:3.18
LABEL maintainer="[e-mail]"
# Run the docker build command below
# Your code here

Hint

The docker build command builds an image from a Dockerfile. Use -t to tag the image with a name and version.

4

List Docker images to verify the build

Run the command docker images mywebapp:1.0 to list the Docker images and confirm that the image mywebapp:1.0 exists.

Docker

docker build -t mywebapp:1.0 .
# Run the docker images command below
# Your code here

Hint

The docker images command shows all images. Filtering by the image name and tag helps confirm your build.

Practice

1. What does the docker build command do?

easy

A. It creates a Docker image from a Dockerfile and context files.

B. It runs a Docker container from an image.

C. It deletes unused Docker images.

D. It lists all running Docker containers.

Solution

1. Step 1: Understand the purpose of docker build

   The docker build command reads instructions from a Dockerfile and packages files from the build context into a new image.

2. Step 2: Differentiate from other Docker commands

   Running containers, deleting images, or listing containers are done by other commands like docker run, docker rmi, and docker ps.

3. Final Answer:

   It creates a Docker image from a Dockerfile and context files. -> Option A

4. Quick Check:

   docker build = create image [OK]

Hint: Remember: build = create image from Dockerfile [OK]

Common Mistakes:

• Confusing build with run command
• Thinking build deletes images
• Mixing build with container listing

2. Which of the following is the correct syntax to build a Docker image named myapp from the current directory?

easy

A. docker build . -name myapp

B. docker build myapp -t .

C. docker build -t myapp .

D. docker build --image myapp .

Solution

1. Step 1: Identify the correct order and options for docker build

   The correct syntax uses -t to tag the image name, followed by the build context (here, current directory .).

2. Step 2: Check each option for syntax correctness

   docker build -t myapp . correctly places -t myapp before the context .. Options A, B, and D misuse flags or order.

3. Final Answer:

   docker build -t myapp . -> Option C

4. Quick Check:

   docker build -t name context [OK]

Hint: Use -t before context folder to name image [OK]

Common Mistakes:

• Placing image name after context
• Using wrong flags like --image or -name
• Omitting the build context folder

3. Given this Dockerfile in the current directory:

FROM alpine
RUN echo "Hello Docker" > /hello.txt

What will be the output of docker build -t testimage . if successful?

medium

A. An image named testimage is created with a file /hello.txt containing 'Hello Docker'.

B. A container runs and prints 'Hello Docker' to the terminal.

C. The build fails because echo is not a valid Docker command.

D. An image is created but /hello.txt is empty.

Solution

1. Step 1: Understand Dockerfile instructions

   FROM alpine sets base image; RUN echo "Hello Docker" > /hello.txt creates a file inside the image.

2. Step 2: Interpret the build command output

   The build creates an image named testimage with the file /hello.txt containing the text 'Hello Docker'. No container runs or prints output during build.

3. Final Answer:

   An image named testimage is created with a file /hello.txt containing 'Hello Docker'. -> Option A

4. Quick Check:

   RUN creates files in image, no output printed [OK]

Hint: RUN commands modify image, they don't print output during build [OK]

Common Mistakes:

• Thinking RUN prints to terminal during build
• Confusing image creation with container run
• Assuming build fails due to shell commands

4. You run docker build -t myapp . but get the error: failed to read dockerfile: open Dockerfile: no such file or directory. What is the most likely cause?

medium

A. The build context folder is empty.

B. The -t option is used incorrectly.

C. Docker daemon is not running.

D. The Dockerfile is missing from the current directory.

Solution

1. Step 1: Analyze the error message

   The error says Docker cannot find the Dockerfile in the build context folder (current directory).

2. Step 2: Identify the cause

   If the Dockerfile is missing, Docker cannot build the image. The -t option and daemon status do not cause this specific error.

3. Final Answer:

   The Dockerfile is missing from the current directory. -> Option D

4. Quick Check:

   Missing Dockerfile = build fails with file not found [OK]

Hint: Check Dockerfile exists in build folder if build fails [OK]

Common Mistakes:

• Assuming wrong flag causes error
• Not verifying Dockerfile presence
• Ignoring daemon status for this error

5. You want to build a Docker image named webapp:v1 using a Dockerfile located in ./app folder, but your current directory is the parent folder. Which command correctly builds the image?

hard

A. docker build -t webapp:v1 ./app

B. docker build -t webapp:v1 -f ./app/Dockerfile ./app

C. docker build ./app -t webapp:v1

D. docker build -f app/Dockerfile -t webapp:v1 ./app

Solution

1. Step 1: Understand build context and Dockerfile location

   The build context is the folder with the Dockerfile and files needed. Here, it's ./app.

2. Step 2: Choose correct command syntax

   docker build -t webapp:v1 -f ./app/Dockerfile ./app uses -t webapp:v1 to tag, -f ./app/Dockerfile to specify Dockerfile path, and ./app as context, which is correct. docker build -t webapp:v1 ./app assumes Dockerfile is in context folder but does not specify path explicitly. docker build ./app -t webapp:v1 has wrong order. docker build -f app/Dockerfile -t webapp:v1 ./app has incomplete Dockerfile path.

3. Final Answer:

   docker build -t webapp:v1 -f ./app/Dockerfile ./app -> Option B

4. Quick Check:

   Tag with -t, specify Dockerfile with -f, context last [OK]

Hint: Tag with -t, specify Dockerfile path with -f, context last [OK]

Common Mistakes:

• Wrong order of options and context
• Incorrect Dockerfile path with -f
• Using current folder when Dockerfile is elsewhere