Image size and minimal base images in Docker - Mini Project: Build & Apply

Image size and minimal base images

📖 Scenario: You are working on a Docker project where keeping the image size small is important for faster downloads and efficient storage. You will create a Dockerfile using a minimal base image and add a simple file to it.

🎯 Goal: Build a Docker image using a minimal base image alpine and add a text file hello.txt with the content Hello, Docker!. Then display the image size using Docker commands.

📋 What You'll Learn

Create a Dockerfile starting with the alpine base image

Add a file named hello.txt with the exact content Hello, Docker!

Use a command to display the size of the built Docker image

💡 Why This Matters

🌍 Real World

Keeping Docker images small helps in faster deployment, less storage use, and quicker updates in real projects.

💼 Career

Understanding minimal base images and image size is essential for DevOps engineers to optimize containerized applications.

1

Create a minimal Dockerfile with alpine base

Create a file named Dockerfile with the first line exactly FROM alpine to use the minimal Alpine Linux base image.

Docker

# Your code here

Hint

The first line in a Dockerfile specifies the base image. Alpine is a very small Linux image.

2

Add a text file with content

Add these two lines to the Dockerfile: RUN echo "Hello, Docker!" > /hello.txt to create the file inside the image.

Docker

FROM alpine
# Your code here

Hint

Use the RUN command to execute shell commands inside the image during build.

3

Build the Docker image with a tag

Write the exact command to build the Docker image with tag hello-alpine using the current directory as context: docker build -t hello-alpine .

Docker

FROM alpine
RUN echo "Hello, Docker!" > /hello.txt
# Your code here

Hint

The docker build command creates the image from the Dockerfile.

4

Display the size of the built image

Write the exact command to show the size of the image named hello-alpine: docker images hello-alpine --format "{{.Repository}}: {{.Size}}"

Docker

FROM alpine
RUN echo "Hello, Docker!" > /hello.txt
# Build command:
docker build -t hello-alpine .
# Your code here

Hint

This command lists images filtered by name and shows their size in a simple format.

Practice

1. Which base image is known for being very small and commonly used to create minimal Docker images?

easy

A. Ubuntu

B. Alpine

C. Debian

D. CentOS

Solution

1. Step 1: Understand common base images

   Ubuntu, Debian, and CentOS are full Linux distributions and tend to be larger in size.

2. Step 2: Identify minimal base images

   Alpine is a lightweight Linux distribution designed for minimal size, often under 10 MB.

3. Final Answer:

   Alpine -> Option B

4. Quick Check:

   Minimal base image = Alpine [OK]

Hint: Alpine is the go-to small base image for minimal Docker images [OK]

Common Mistakes:

• Choosing Ubuntu or Debian as minimal base images
• Confusing full distributions with minimal ones
• Not knowing Alpine is designed for small size

2. Which of the following Dockerfile base image declarations is syntactically correct for using Alpine Linux?

easy

A. FROM alpine:latest

B. BASE alpine

C. USE alpine

D. IMAGE alpine

Solution

1. Step 1: Recall Dockerfile syntax for base images

   The correct keyword to specify a base image is FROM followed by the image name and optional tag.

2. Step 2: Check each option

   Only 'FROM alpine:latest' uses the correct syntax. Others are invalid Dockerfile commands.

3. Final Answer:

   FROM alpine:latest -> Option A

4. Quick Check:

   Base image syntax = FROM [OK]

Hint: Base images start with FROM in Dockerfiles [OK]

Common Mistakes:

• Using incorrect keywords like BASE or USE
• Omitting the FROM keyword
• Not specifying image tag correctly

3. Given this Dockerfile snippet:

FROM alpine:3.18
RUN apk add --no-cache curl

What is the main reason this image remains small compared to using Ubuntu as base?

medium

A. Alpine automatically removes all packages after installation

B. Alpine installs curl faster than Ubuntu

C. Ubuntu images do not support curl installation

D. Alpine uses apk with --no-cache to avoid cache files, reducing size

Solution

1. Step 1: Understand Alpine's package manager behavior

   Alpine uses apk, and the --no-cache option prevents storing temporary cache files, keeping the image small.

2. Step 2: Compare with Ubuntu

   Ubuntu uses apt which caches package files unless explicitly cleaned, increasing image size.

3. Final Answer:

   Alpine uses apk with --no-cache to avoid cache files, reducing size -> Option D

4. Quick Check:

   apk --no-cache reduces image size [OK]

Hint: Use apk add --no-cache on Alpine to keep images small [OK]

Common Mistakes:

• Thinking Alpine installs packages faster
• Believing Ubuntu can't install curl
• Assuming Alpine removes all packages automatically

4. You wrote this Dockerfile:

FROM alpine
RUN apk add curl

But your final image is much larger than expected. What is the likely cause?

medium

A. You forgot to use --no-cache with apk add, so cache files remain

B. Alpine base image is always large regardless of commands

C. You should use Ubuntu base image for smaller size

D. The RUN command syntax is incorrect

Solution

1. Step 1: Analyze apk add command

   Without --no-cache, apk stores cache files in the image, increasing size.

2. Step 2: Understand base image size and syntax

   Alpine is small by default, RUN syntax is correct, and Ubuntu is larger, so those are not causes.

3. Final Answer:

   You forgot to use --no-cache with apk add, so cache files remain -> Option A

4. Quick Check:

   Missing --no-cache causes larger images [OK]

Hint: Always add --no-cache with apk to keep Alpine images small [OK]

Common Mistakes:

• Assuming Alpine is large by default
• Using wrong RUN syntax
• Switching to Ubuntu thinking it is smaller

5. You want to create the smallest possible Docker image for a Go application. Which base image and approach is best to minimize image size?

hard

A. Use FROM ubuntu and install Go runtime

B. Use FROM alpine and install full Go toolchain

C. Use FROM scratch and copy only the compiled Go binary

D. Use FROM debian and add all dependencies

Solution

1. Step 1: Understand the scratch base image

   Scratch is an empty image with zero size, ideal for copying only necessary files like a compiled binary.

2. Step 2: Compare other options

   Ubuntu, Alpine, and Debian add extra layers and packages, increasing image size unnecessarily.

3. Final Answer:

   Use FROM scratch and copy only the compiled Go binary -> Option C

4. Quick Check:

   Scratch + compiled binary = smallest image [OK]

Hint: Use scratch base and copy binary for tiniest Go images [OK]

Common Mistakes:

• Installing full runtimes increasing image size
• Using larger base images unnecessarily
• Not using multi-stage builds to compile separately