docker-compose.yml structure - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of docker-compose up with multiple services

Given this docker-compose.yml file, what will be the output when running docker-compose up?

Docker

version: '3.8'
services:
  web:
    image: nginx:alpine
    ports:
      - "8080:80"
  db:
    image: postgres:13
    environment:
      POSTGRES_PASSWORD: example

A Starts two containers: one running nginx accessible on port 8080, and one running postgres with password set

B Starts one container combining nginx and postgres in a single service

C Fails with error: 'image not found' because alpine tag is invalid

D Starts only the nginx container; postgres is ignored

💡 Hint

Think about how docker-compose handles multiple services defined under 'services'.

docker-compose starts each service as a separate container. The 'web' service runs nginx on port 8080, and the 'db' service runs postgres with the specified environment variable.

❓ Configuration

intermediate

Correct syntax for environment variables in docker-compose.yml

Which option shows the correct way to set environment variables for a service in docker-compose.yml?

A

environment:
  - POSTGRES_PASSWORD: example
  - POSTGRES_USER: admin

B

environment:
  POSTGRES_PASSWORD: example
  POSTGRES_USER: admin

C

environment:
  POSTGRES_PASSWORD=example
  POSTGRES_USER=admin

D

environment:
  - POSTGRES_PASSWORD=example
  - POSTGRES_USER=admin

💡 Hint

Look for the YAML mapping style used for environment variables.

In docker-compose.yml, environment variables can be set as a mapping (key: value) or as a list of strings with key=value. Option B uses mapping correctly.

❓ Troubleshoot

advanced

Error caused by incorrect indentation in docker-compose.yml

What error will occur if the following docker-compose.yml snippet is used?

Docker

version: '3.8'
services:
  web:
  image: nginx:alpine
  ports:
    - "8080:80"

A docker-compose fails with 'image not found' error

B docker-compose runs successfully with no errors

C docker-compose ignores the 'ports' section silently

D yaml.parser.ParserError due to incorrect indentation under 'web'

💡 Hint

YAML is sensitive to spaces and indentation.

The 'image' and 'ports' keys must be indented under 'web'. Incorrect indentation causes a YAML parsing error.

🔀 Workflow

advanced

Order of steps to add a new service to an existing docker-compose.yml

What is the correct order of steps to add a new service called 'cache' running Redis to an existing docker-compose.yml and start it?

A 1,4,2,3

B 4,1,2,3

C 1,2,4,3

D 2,1,4,3

💡 Hint

Think about editing the file before starting services and verifying after.

First add the service, then add ports if needed, then start services, then verify running containers.

✅ Best Practice

expert

Choosing the best way to share environment variables securely in docker-compose.yml

Which option is the best practice to securely provide environment variables like passwords to services in docker-compose.yml?

A Hardcode passwords directly in the environment section of docker-compose.yml

B Pass passwords as command line arguments when running docker-compose up

C Use an external .env file and reference variables in docker-compose.yml

D Store passwords in the image and do not specify in docker-compose.yml

💡 Hint

Think about separating secrets from code files.

Using an external .env file keeps secrets out of the main compose file and allows better security and flexibility.