Reducing image size strategies in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a common way to reduce Docker image size by choosing the base image?

Use a minimal base image like Alpine Linux instead of a full OS image. Alpine is very small and includes only essential packages.

beginner

How does combining RUN commands in a Dockerfile help reduce image size?

Combining multiple RUN commands into one reduces the number of layers, which lowers the final image size.

intermediate

Why should you remove package manager caches and temporary files in Docker images?

Removing caches and temp files after installing packages prevents unnecessary data from staying in the image, making it smaller.

intermediate

What is a multi-stage build in Docker and how does it help reduce image size?

Multi-stage builds use multiple FROM statements to build in stages. You copy only the final needed artifacts to the last stage, leaving behind build tools and files, reducing image size.

beginner

How can using .dockerignore file help in reducing Docker image size?

The .dockerignore file excludes unnecessary files and folders from the build context, so they are not copied into the image, keeping it smaller.

Which base image is known for being very small and commonly used to reduce Docker image size?

A CentOS

B Ubuntu

C Alpine

D Debian

Alpine is a minimal Linux distribution designed to be small and secure, making it popular for small Docker images.

What is the benefit of combining multiple RUN commands into a single RUN command in a Dockerfile?

A Reduces the number of image layers

B Speeds up container startup time

C Increases image caching

D Improves network security

Combining RUN commands reduces the number of layers, which helps reduce the final image size.

What does a multi-stage Docker build allow you to do?

A Compress images after build

B Run multiple containers from one image

C Automatically update base images

D Build images in stages and copy only needed files to the final image

Multi-stage builds let you separate build and runtime environments, copying only necessary files to keep the final image small.

Why should you clean up package manager caches in a Dockerfile?

A To speed up package installation

B To reduce image size by removing unnecessary files

C To improve container security

D To enable automatic updates

Package manager caches are temporary files that increase image size if not removed.

What is the purpose of the .dockerignore file?

A Exclude files from the build context to reduce image size

B Specify environment variables for the container

C Define network settings for the container

D List commands to run after container start

The .dockerignore file tells Docker which files to ignore when sending the build context, helping reduce image size.

Explain three strategies to reduce Docker image size and why they work.

Think about base image choice, Dockerfile commands, and cleanup steps.

Describe how multi-stage builds help in creating smaller Docker images.

Focus on separating build environment from final image.

Practice

1. Which of the following is the best initial step to reduce the size of a Docker image?

easy

A. Start with a small base image like Alpine or slim variants

B. Install all available packages to avoid missing dependencies

C. Use the latest Ubuntu image without modifications

D. Add unnecessary files to the image for future use

Solution

1. Step 1: Understand base image impact

   The base image sets the starting size; smaller bases mean smaller final images.

2. Step 2: Compare options

   Alpine and slim variants are minimal, unlike full Ubuntu or adding unnecessary files.

3. Final Answer:

   Start with a small base image like Alpine or slim variants -> Option A

4. Quick Check:

   Small base image = smaller Docker image [OK]

Hint: Choose minimal base images first to reduce size [OK]

Common Mistakes:

• Using full OS images without slimming
• Installing unnecessary packages
• Adding extra files without need

2. Which Dockerfile command sequence correctly cleans up package manager caches to reduce image size?

easy

A. RUN apt-get update && rm -rf /var/lib/apt/lists/* && apt-get install curl

B. RUN apt-get install curl && apt-get update

C. RUN rm -rf /var/lib/apt/lists/* && apt-get install curl

D. RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

Solution

1. Step 1: Understand package installation order

   First update package lists, then install packages, finally clean cache to reduce image size.

2. Step 2: Analyze commands

   RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/* updates, installs, then removes cache correctly; others have wrong order causing errors or no cleanup.

3. Final Answer:

   RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/* -> Option D

4. Quick Check:

   Update -> install -> clean cache = correct order [OK]

Hint: Update before install, clean cache after install [OK]

Common Mistakes:

• Cleaning cache before installing packages
• Skipping apt-get update
• Wrong command order causing errors

3. Given this Dockerfile snippet, what will be the approximate size effect on the final image?

FROM node:18-alpine
RUN apk add --no-cache git
COPY . /app
RUN npm install

medium

A. The image will be small because Alpine and --no-cache reduce size

B. The image will be large because npm install always adds big size

C. The image will be large because git is installed without cache cleanup

D. The image size is unaffected by base image or package installs

Solution

1. Step 1: Analyze base image and package install

   Using Alpine base and apk with --no-cache avoids cache files, keeping image small.

2. Step 2: Consider npm install impact

   npm install adds dependencies but starting from Alpine keeps overall size smaller than heavier bases.

3. Final Answer:

   The image will be small because Alpine and --no-cache reduce size -> Option A

4. Quick Check:

   Alpine + --no-cache = smaller image [OK]

Hint: Use Alpine + --no-cache to keep image small [OK]

Common Mistakes:

• Assuming npm install always makes image large
• Ignoring --no-cache effect
• Not considering base image size

4. You wrote this Dockerfile to reduce image size but the final image is still large:

FROM python:3.12
RUN pip install flask
RUN rm -rf /root/.cache/pip

What is the main issue causing the large image?

medium

A. The pip cache is not cleaned because it is stored elsewhere

B. You should combine RUN commands to reduce layers

C. The base image python:3.12 is already large and cannot be reduced

D. Flask installation always creates large images regardless

Solution

1. Step 1: Understand Docker layers

   Each RUN creates a new layer; cleaning cache in a separate RUN keeps cache in previous layer.

2. Step 2: Combine commands to remove cache in same layer

   Combining pip install and cache removal in one RUN prevents cache from persisting in earlier layers.

3. Final Answer:

   You should combine RUN commands to reduce layers -> Option B

4. Quick Check:

   Combine RUN commands to clean caches in one layer [OK]

Hint: Combine install and cleanup in one RUN to reduce layers [OK]

Common Mistakes:

• Cleaning cache in separate RUN commands
• Assuming base image size can't be optimized
• Believing package size can't be reduced

5. You want to build a Docker image for a Go application but keep it as small as possible. Which multi-stage Dockerfile snippet achieves this best?

FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp

FROM alpine:latest
COPY --from=builder /app/myapp /usr/local/bin/myapp
CMD ["myapp"]

hard

A. This build copies all source files to Alpine, increasing image size unnecessarily

B. Using Alpine as final image is bad because it lacks Go runtime

C. This multi-stage build compiles in golang image and copies only binary to Alpine, minimizing size

D. You should install Go in Alpine to build and run in one stage

Solution

1. Step 1: Understand multi-stage build purpose

   Build stage uses full Go image; final stage uses minimal Alpine with only binary copied.

2. Step 2: Analyze final image contents

   Only the compiled binary is copied, no source or build tools, keeping image small.

3. Final Answer:

   This multi-stage build compiles in golang image and copies only binary to Alpine, minimizing size -> Option C

4. Quick Check:

   Multi-stage build + minimal final image = small size [OK]

Hint: Use multi-stage builds to copy only final artifacts [OK]

Common Mistakes:

• Copying source files to final image
• Installing build tools in final image
• Building and running in same large image