Reducing image size strategies in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use multi-stage builds in Docker?

What is the main reason to use multi-stage builds when creating Docker images?

A To run multiple containers from one Dockerfile simultaneously

B To separate build dependencies from runtime, reducing final image size

C To speed up the container startup time by caching layers

D To allow Docker images to run on different operating systems automatically

💡 Hint

Think about how build tools and runtime requirements differ.

Multi-stage builds let you copy only the necessary artifacts from a build stage to the final image, leaving behind bulky build tools and dependencies. This reduces the final image size.

💻 Command Output

intermediate

Output of Docker image size after cleaning cache

What will be the effect on the Docker image size after running RUN apt-get clean && rm -rf /var/lib/apt/lists/* in a Dockerfile?

Docker

FROM ubuntu:22.04
RUN apt-get update && apt-get install -y curl \
    && apt-get clean && rm -rf /var/lib/apt/lists/*

A The image size increases due to added cleanup commands

B No change in image size because cache files are stored outside the image

C The image size decreases because package cache files are removed

D Build will fail because rm command is invalid in Dockerfile

💡 Hint

Consider where package cache files are stored and if they remain in the image layers.

Cleaning the apt cache and removing package lists deletes unnecessary files from the image layers, reducing the final image size.

❓ Configuration

advanced

Choosing base images for smaller Docker images

Which base image choice will generally produce the smallest Docker image for a Node.js app?

A node:alpine

B node:latest

C ubuntu:22.04

D debian:stable-slim

💡 Hint

Think about minimal Linux distributions optimized for small size.

Alpine Linux is a minimal distribution designed for small size, so node:alpine images are much smaller than full Ubuntu or Debian base images.

❓ Troubleshoot

advanced

Why does the Docker image remain large despite cleanup commands?

You added cleanup commands to your Dockerfile to remove temporary files after installing packages, but the final image size is still large. What is the most likely reason?

A The Dockerfile must use COPY instead of RUN for cleanup to work

B Docker caches all files permanently regardless of cleanup commands

C Cleanup commands only work on Alpine images, not Ubuntu-based ones

D Each RUN command creates a new layer; cleanup must be in the same RUN as install

💡 Hint

Think about how Docker layers work and when files get saved.

Docker creates a new image layer after each RUN command. If cleanup is done in a separate RUN, the files exist in previous layers and still increase image size. Combining install and cleanup in one RUN keeps the image smaller.

🔀 Workflow

expert

Optimizing Dockerfile for smallest image with build artifacts

You have a Dockerfile that compiles a Go application and then copies the binary to a minimal image. Which multi-stage Dockerfile snippet correctly produces the smallest final image?

Docker

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

FROM scratch
COPY --from=builder /app/myapp /myapp
CMD ["/myapp"]

A Use <code>FROM scratch</code> as final stage and copy only the binary from builder

B Use <code>FROM golang:1.20</code> as final stage and copy all files from builder

C Use <code>FROM ubuntu</code> as final stage and copy the binary and source files

D Use <code>FROM alpine</code> as final stage and copy the entire /app directory

💡 Hint

Consider the smallest base image possible and copying only what is needed.

Using scratch as the final base image and copying only the compiled binary results in the smallest image. Other options include unnecessary files or larger base images.