Image size and minimal base images in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a minimal base image in Docker?

A minimal base image is a very small Docker image that contains only the essential files needed to run applications. It helps reduce the overall image size and improves efficiency.

beginner

Why is keeping Docker image size small important?

Smaller Docker images use less disk space, download faster, and start containers quicker. This saves bandwidth and improves deployment speed.

intermediate

Name two popular minimal base images used in Docker.

Two popular minimal base images are Alpine Linux and BusyBox. Both are very small and commonly used to build lightweight containers.

intermediate

How can you reduce Docker image size when building your own image?

You can reduce image size by using minimal base images, removing unnecessary files, combining commands in one layer, and cleaning up package caches.

advanced

What is the trade-off when using very minimal base images like Alpine?

Minimal base images may lack some libraries or tools, so you might need to install extra packages. This can add complexity but keeps the image small.

Which of these is a minimal base image commonly used in Docker?

A Alpine

B Ubuntu Desktop

C Windows Server

D CentOS Full

Alpine is a very small Linux distribution often used as a minimal base image in Docker.

Why should you keep Docker images small?

A To save disk space and speed up downloads

B To make containers run slower

C To increase image complexity

D To use more bandwidth

Smaller images save disk space and download faster, improving deployment speed.

What is a common method to reduce Docker image size?

A Avoid cleaning package caches

B Use minimal base images

C Keep all build files inside the image

D Add more layers

Using minimal base images helps reduce the overall image size.

What might you need to do when using a minimal base image like Alpine?

A Use Windows commands

B Add a graphical user interface

C Increase image size intentionally

D Install missing libraries or tools

Minimal images may lack some tools, so you often install needed packages.

Which command helps reduce image layers in a Dockerfile?

A Use multiple RUN commands separately

B Add unnecessary files

C Combine multiple commands with && in one RUN

D Use COPY multiple times

Combining commands with && in one RUN reduces the number of layers and image size.

Explain what a minimal base image is and why it is useful in Docker.

Think about the smallest starting point for your container.

Describe at least three ways to reduce Docker image size when building your own images.

Focus on Dockerfile best practices and cleanup.

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