Docker Compose for dev environment - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the version of Docker Compose file format.

Docker

version: '[1]'
services:
  app:
    image: myapp:latest

Drag options to blanks, or click blank then click option'

A4

B3.8

C1

D2.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using a numeric value without quotes causes syntax errors.

Choosing an unsupported version number.

The version '3.8' is a widely used and stable Docker Compose file format version suitable for development environments.

2fill in blank

medium

Complete the code to expose port 8000 on the host machine.

Docker

services:
  web:
    image: mywebapp
    ports:
      - '[1]:8000'

Drag options to blanks, or click blank then click option'

A443

B80

C8080

D8000

Attempts:

3 left

💡 Hint

Common Mistakes

Using a port number already in use on the host.

Swapping host and container ports.

Mapping port 8000 on the host to port 8000 in the container allows access to the app on localhost:8000.

3fill in blank

hard

Fix the error in the volume mount to map the current directory to /app inside the container.

Docker

services:
  backend:
    image: backend:dev
    volumes:
      - '[1]:/app'

Drag options to blanks, or click blank then click option'

A./

B/app

C.

D./src

Attempts:

3 left

💡 Hint

Common Mistakes

Using absolute paths that don't exist on the host.

Using incorrect relative paths like '.' without slash.

Using './' correctly maps the current directory to /app in the container, which is common in dev setups.

4fill in blank

hard

Fill both blanks to set environment variables for the service.

Docker

services:
  db:
    image: postgres
    environment:
      - [1]=devuser
      - [2]=devpass

Drag options to blanks, or click blank then click option'

APOSTGRES_USER

BPOSTGRES_PASSWORD

CDB_USER

DDB_PASS

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic variable names that Postgres does not recognize.

Swapping user and password variable names.

Postgres expects environment variables POSTGRES_USER and POSTGRES_PASSWORD to set up the database user and password.

5fill in blank

hard

Fill all three blanks to define a service with build context, restart policy, and command override.

Docker

services:
  api:
    build: [1]
    restart: [2]
    command: [3]

Drag options to blanks, or click blank then click option'

A./api

Balways

C"python app.py"

Dnever

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect paths for build context.

Choosing wrong restart policies like 'never' for dev environments.

Not quoting the command string properly.

The build context './api' points to the Dockerfile location. Restart policy 'always' keeps the container running. The command overrides the default to run the app.

Practice

1. What is the main purpose of using docker-compose.yml in a development environment?

easy

A. To build Docker images only

B. To define and run multiple containers with one command

C. To replace Docker CLI commands completely

D. To monitor container resource usage

Solution

1. Step 1: Understand Docker Compose role

   Docker Compose uses a YAML file to define multiple containers and their settings.

2. Step 2: Identify main benefit in dev

   It allows running all containers together with a single command, simplifying setup.

3. Final Answer:

   To define and run multiple containers with one command -> Option B

4. Quick Check:

   Compose = multiple containers, one command [OK]

Hint: Compose runs many containers with one file and command [OK]

Common Mistakes:

• Thinking Compose only builds images
• Believing Compose replaces all Docker CLI commands
• Confusing Compose with monitoring tools

2. Which of the following is the correct syntax to expose port 8080 on the host mapped to port 80 in a docker-compose.yml service?

easy

A. ports:
- "80:8080"

B. expose:
- "80:8080"

C. expose:
- "8080:80"

D. ports:
- "8080:80"

Solution

1. Step 1: Understand ports syntax in Compose

   The format is "host_port:container_port" to map host port to container port.

2. Step 2: Match ports for host 8080 to container 80

   To expose container port 80 on host port 8080, use "8080:80" under ports.

3. Final Answer:

   ports: - "8080:80" -> Option D

4. Quick Check:

   HostPort:ContainerPort = 8080:80 [OK]

Hint: Ports use host:container format [OK]

Common Mistakes:

• Reversing host and container ports
• Using expose instead of ports for host mapping
• Incorrect YAML indentation

3. Given this docker-compose.yml snippet, what will be the effect of the volume configuration?

services:
  app:
    image: myapp
    volumes:
      - ./src:/app/src

medium

A. The host's ./src folder is shared with /app/src inside the container

B. The container's /app/src folder is copied to host's ./src folder once

C. The container will have an empty /app/src folder

D. The volume will cause a syntax error

Solution

1. Step 1: Understand volume syntax in Compose

   Format "host_path:container_path" mounts host folder inside container.

2. Step 2: Analyze given volume mapping

   Host folder ./src is mounted to container path /app/src, sharing files live.

3. Final Answer:

   The host's ./src folder is shared with /app/src inside the container -> Option A

4. Quick Check:

   Volume mount = host folder shared inside container [OK]

Hint: Volumes share host folder inside container path [OK]

Common Mistakes:

• Thinking volume copies files only once
• Confusing source and target paths
• Assuming volume creates empty folder

4. You wrote this docker-compose.yml but get an error when running docker-compose up. What is wrong?

services:
  web:
    image: nginx
    ports:
      - "80:80"
    volumes:
      - ./html:/usr/share/nginx/html
    environment:
      - DEBUG=true
    command: nginx -g 'daemon off;'
    restart: always
  db:
    image: postgres
    environment:
      POSTGRES_PASSWORD: example
    volumes:
      - pgdata:/var/lib/postgresql/data
volumes:
  pgdata:

medium

A. The environment variable for db should use list syntax or key-value mapping consistently

B. The volume pgdata is not defined properly under volumes

C. The command for web service is invalid

D. The environment variable for db uses wrong syntax, should be a list

Solution

1. Step 1: Inspect YAML syntax in command field

   The command: nginx -g 'daemon off;' lacks quotes around the full string, causing YAML to parse it as separate unquoted scalars, leading to a syntax error.

2. Step 2: Confirm other sections are valid

   Environment uses valid list/map styles (mixed OK), volumes properly defined.

3. Final Answer:

   The command for web service is invalid -> Option C

4. Quick Check:

   Multi-word command needs full quotes or YAML list [OK]

Hint: YAML command with spaces: quote whole or use list [OK]

Common Mistakes:

• Blaming environment syntax mixing (both styles valid)
• Misreading pgdata volume definition
• Overlooking unquoted multi-word command

5. You want to set up a development environment with a web app and a database using Docker Compose. The web app needs live code updates from your host machine, and the database should persist data between restarts. Which docker-compose.yml configuration correctly achieves this?

hard

A. services:
web:
image: mywebapp
volumes:
- ./code:/app
ports:
- "3000:3000"
db:
image: mysql
environment:
MYSQL_ROOT_PASSWORD: rootpass
volumes:
- dbdata:/var/lib/mysql
volumes:
dbdata:

B. services:
web:
image: mywebapp
ports:
- "3000:3000"
db:
image: mysql
environment:
MYSQL_ROOT_PASSWORD: rootpass
volumes:
dbdata:
driver: local

C. services:
web:
image: mywebapp
volumes:
- /app:/code
ports:
- "3000:3000"
db:
image: mysql
environment:
MYSQL_ROOT_PASSWORD: rootpass
volumes:
- dbdata:/var/lib/mysql
volumes:
dbdata:

D. services:
web:
image: mywebapp
volumes:
- ./code:/app
ports:
- "3000:3000"
db:
image: mysql
environment:
MYSQL_ROOT_PASSWORD: rootpass

Solution

1. Step 1: Ensure live code updates for web app

   Mount host folder ./code to container /app using volumes for live updates.

2. Step 2: Ensure database data persistence

   Use a named volume (dbdata) mounted to /var/lib/mysql to keep data between restarts.

3. Step 3: Check ports and environment

   Expose port 3000 for web and set MYSQL_ROOT_PASSWORD for db environment.

4. Final Answer:

   Configuration with volumes for web and named volume for db -> Option A

5. Quick Check:

   Volumes for live code and persistent db = services: web: image: mywebapp volumes: - ./code:/app ports: - "3000:3000" db: image: mysql environment: MYSQL_ROOT_PASSWORD: rootpass volumes: - dbdata:/var/lib/mysql volumes: dbdata: [OK]

Hint: Use volumes for code and named volume for DB persistence [OK]

Common Mistakes:

• Not mounting host code folder for live updates
• Missing volume for database persistence
• Incorrect volume paths or missing volumes section