Multiple Compose files (override) in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Docker Compose with override file

Given two Docker Compose files, docker-compose.yml and docker-compose.override.yml, what will be the value of the environment variable MODE in the running container?

docker-compose.yml:

services:
  app:
    image: myapp:latest
    environment:
      - MODE=production

docker-compose.override.yml:

services:
  app:
    environment:
      - MODE=development

Command run: docker compose up

A MODE=development

B MODE=production,MODE=development (both set)

C MODE=production

D No MODE environment variable set

💡 Hint

Override files merge and override keys, environment variables in override replace the original.

Docker Compose merges the override file on top of the base file. The environment variable MODE in the override file replaces the one in the base file, so the container runs with MODE=development.

❓ Configuration

intermediate

Correct syntax to override ports in Compose override file

You have a base docker-compose.yml with a service exposing port 80. You want to override it in docker-compose.override.yml to expose port 8080 instead. Which override snippet correctly replaces the port mapping?

A

services:
  web:
    ports:
      - 8080:80

B

services:
  web:
    ports:
      - "8080:80"

C

services:
  web:
    ports:
      - 80:8080

D

services:
  web:
    expose:
      - "8080"

💡 Hint

Ports must be strings with quotes and format hostPort:containerPort.

The correct syntax uses quotes around the port mapping string and specifies host port first, then container port. Option B correctly overrides the port mapping to expose container port 80 on host port 8080.

❓ Troubleshoot

advanced

Why does the override file not change the command?

You have this base Compose file snippet:

services:
  app:
    image: myapp
    command: run-server

And this override snippet:

services:
  app:
    command: run-debug

After running docker compose up, the container still runs run-server. What is the most likely reason?

A The override file is not named correctly or not detected by Docker Compose

B The command key cannot be overridden in Compose override files

C Docker Compose caches the command and ignores overrides

D The image 'myapp' has an entrypoint that ignores the command

💡 Hint

Check the file names and how Docker Compose loads override files automatically.

Docker Compose automatically loads docker-compose.override.yml if it exists. If the override file is named differently or not in the current directory, it won't be applied, so the command remains unchanged.

🔀 Workflow

advanced

Using multiple override files in Docker Compose

You want to run Docker Compose with two override files: docker-compose.override.yml and docker-compose.prod.yml. Which command correctly applies both overrides with the base docker-compose.yml?

A docker compose -f docker-compose.override.yml -f docker-compose.prod.yml up

B docker compose up -f docker-compose.override.yml -f docker-compose.prod.yml

C docker compose -f docker-compose.yml -f docker-compose.override.yml -f docker-compose.prod.yml up

D docker compose up --override-files docker-compose.override.yml docker-compose.prod.yml

💡 Hint

Use multiple -f flags with the base file first, then overrides in order.

Docker Compose merges files in the order they are specified with -f flags. The base file must be first, followed by override files. Option C correctly specifies all three files in order.

✅ Best Practice

expert

Best practice for sensitive data in multiple Compose files

You have a base Compose file and an override file for production. You need to keep sensitive environment variables out of version control but still use them in production. What is the best practice?

A Pass sensitive variables as command line arguments when running docker compose up

B Hardcode sensitive variables directly in the override Compose file for production

C Include sensitive variables in the base Compose file but encrypt the file in version control

D Store sensitive variables in a separate .env file and reference it in the override file, which is excluded from version control

💡 Hint

Use environment files and .gitignore to keep secrets safe.

The best practice is to keep sensitive data in a separate .env file that is referenced by the override Compose file and excluded from version control. This keeps secrets out of code and allows safe overrides.