FROM instruction for base image in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Docker build with FROM alpine

What will be the base image used and the first line output when building this Dockerfile?

FROM alpine
RUN echo Hello World

Docker

FROM alpine
RUN echo Hello World

A

Base image: alpine
Output: Hello World

B

Base image: ubuntu
Output: Hello World

C

Base image: alpine
Output: RUN echo Hello World

D

Base image: scratch
Output: Hello World

💡 Hint

The FROM instruction sets the base image for the build.

The FROM alpine sets the base image to alpine. The RUN command executes and prints Hello World.

🧠 Conceptual

intermediate

Purpose of the FROM instruction

What is the main purpose of the FROM instruction in a Dockerfile?

A To copy files into the image

B To specify the base image for the Docker build

C To run commands inside the container

D To expose ports from the container

💡 Hint

Think about what sets the starting point for building an image.

FROM sets the base image which is the starting point for the Docker image build.

❓ Configuration

advanced

Using multiple FROM instructions

What happens when a Dockerfile contains multiple FROM instructions?

A Only the first FROM is used; others are ignored

B The last FROM overrides all previous ones

C Docker throws a syntax error

D Each FROM starts a new build stage, allowing multi-stage builds

💡 Hint

Multiple FROM instructions enable building in stages.

Multiple FROM instructions create separate build stages, useful for multi-stage builds.

❓ Troubleshoot

advanced

Error when using invalid base image in FROM

What error will Docker show if the base image specified in FROM does not exist?

A manifest for <image> not found: manifest unknown: manifest unknown

B Syntax error near FROM instruction

C Permission denied while accessing base image

D Container exited with code 1

💡 Hint

Docker tries to pull the image and fails if it doesn't exist.

Docker reports manifest not found error when the base image is missing.

✅ Best Practice

expert

Choosing a base image for minimal size

Which base image is best to use in the FROM instruction for the smallest possible image size?

A debian

B ubuntu

C scratch

D centos

💡 Hint

One base image is completely empty and minimal.

scratch is an empty image with zero size, ideal for minimal images.

Practice

1. What does the FROM instruction do in a Dockerfile?

easy

A. It sets the base image for the Docker build.

B. It copies files from the host to the container.

C. It runs commands inside the container.

D. It exposes ports for the container.

Solution

1. Step 1: Understand the role of FROM

   The FROM instruction defines the starting point or base image for building a Docker image.

2. Step 2: Compare with other instructions

   Other instructions like COPY, RUN, and EXPOSE have different purposes, not setting the base image.

3. Final Answer:

   It sets the base image for the Docker build. -> Option A

4. Quick Check:

   FROM = base image [OK]

Hint: FROM always sets the base image in Dockerfile [OK]

Common Mistakes:

• Confusing FROM with COPY or RUN instructions
• Thinking FROM exposes ports
• Assuming FROM copies files

2. Which of the following is the correct syntax for the FROM instruction to use the Ubuntu 22.04 image?

easy

A. FROM ubuntu/22.04

B. FROM ubuntu-22.04

C. FROM ubuntu:22.04

D. FROM ubuntu@22.04

Solution

1. Step 1: Recall Docker image tag syntax

   The correct format is image_name:tag, so ubuntu:22.04 is valid.

2. Step 2: Check other options

   Options with dashes, slashes, or @ are invalid for tags in FROM instruction.

3. Final Answer:

   FROM ubuntu:22.04 -> Option C

4. Quick Check:

   Use colon for tag in FROM [OK]

Hint: Use colon ':' to specify image tag in FROM [OK]

Common Mistakes:

• Using dash '-' instead of colon ':' for tags
• Using slash '/' incorrectly in image name
• Using '@' instead of ':' for tags

3. Given this Dockerfile snippet:

FROM python:3.12-slim
RUN python --version

What will be the output when building this Docker image?

medium

A. Python 3.11.x

B. Python 3.12.x

C. Error: python command not found

D. Python 2.7.x

Solution

1. Step 1: Identify base image version

   The base image is python:3.12-slim, which includes Python version 3.12.

2. Step 2: Understand RUN command output

   The RUN command executes python --version, which prints the Python version installed in the image.

3. Final Answer:

   Python 3.12.x -> Option B

4. Quick Check:

   FROM python:3.12-slim runs Python 3.12 [OK]

Hint: Tag in FROM matches Python version output [OK]

Common Mistakes:

• Assuming default Python version is 3.11 or 2.7
• Thinking slim images lack Python
• Confusing build output with runtime

4. Identify the error in this Dockerfile snippet:

FROM alpine
FROM ubuntu:20.04
RUN echo "Hello"

medium

A. FROM instruction must be last line.

B. Missing tag for alpine image causes error.

C. Multiple FROM instructions without stage names cause error.

D. There is no error in this Dockerfile snippet.

Solution

1. Step 1: Check multiple FROM usage

   Multiple FROM instructions are allowed for multi-stage builds even without stage names (AS). Earlier stages are discarded unless referenced (by name or index).

2. Step 2: Verify other parts

   FROM alpine defaults to :latest (valid image). The RUN applies to the final stage (ubuntu:20.04) and is syntactically correct.

3. Final Answer:

   There is no error in this Dockerfile snippet. -> Option D

4. Quick Check:

   Multiple FROM without AS is valid [OK]

Hint: Multiple FROM instructions do not require stage names [OK]

Common Mistakes:

• Assuming multiple FROM lines always work without names
• Thinking RUN syntax is wrong here
• Believing FROM must be last line

5. You want to build a multi-stage Dockerfile using node:18 as the build stage (named 'build') and nginx:alpine as the final stage. Which FROM instructions correctly set this up?

hard

A. FROM node:18 AS build
FROM nginx:alpine

B. FROM node:18
FROM nginx:alpine AS build

C. FROM node:18
FROM nginx:alpine

D. FROM node:18 AS build
FROM nginx:alpine AS build

Solution

1. Step 1: Understand multi-stage build syntax

   Use AS to name the build stage, e.g., FROM node:18 AS build.

2. Step 2: Set final stage without conflicting names

   The final stage uses FROM nginx:alpine without a stage name or with a different name.

3. Step 3: Check options

   FROM node:18 AS build FROM nginx:alpine correctly names the build stage and then starts the final stage. Others misuse stage names or omit them incorrectly.

4. Final Answer:

   FROM node:18 AS build FROM nginx:alpine -> Option A

5. Quick Check:

   Use AS to name build stage first [OK]

Hint: Name build stage with AS, final stage without or different name [OK]

Common Mistakes:

• Naming both stages the same
• Placing AS on final stage incorrectly
• Omitting AS for build stage in multi-stage