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Docker Intermediate: Images and Compose That Survive Real Projects
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Docker Intermediate: Images and Compose That Survive Real Projects

Check Yourself: Compose for Real Apps

Five scenario questions on depends_on, health checks, job dependencies, env files and secrets. If you miss more than one, reread the lessons in this section.

5 min DockerSecurityDeployment and ContainersPackaging and Dependencies Intermediate
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1. Your Compose file has depends_on with db in the short form. The app still fails on its first connection, and works when you start it again a minute later. Why?
depends_on is ignored by Compose version 2. The short form waits for the db container to start, not for the database inside it to be ready. Add a health check and condition service_healthy. The app needs a longer restart delay set in the Dockerfile. The database image is too old.
2. A health check uses curl, but the image only has wget. The container never becomes healthy and the service that depends on it never starts. What is the cause?
The interval is too long. Health checks only work with the CMD-SHELL form. The check command does not exist in the image, so it fails every time and the container ends up unhealthy. Compose cannot run health checks for nginx.
3. A migrate service runs once and exits with code 0. Which depends_on condition makes another service wait until the migration has finished successfully?
service_completed_successfully No condition is needed, because depends_on waits for exit by default. service_healthy service_started
4. Your .env file contains GREETING=hello, and the Compose file has no mention of GREETING. Does the container see GREETING?
Yes, but only after docker compose config is run. No. The .env file only fills placeholders in the Compose file. Use env_file or environment to put variables into the container. Only if the variable name is written in capital letters. Yes, every variable in .env is passed to every container.
5. You need a database password available to a container, and you do not want it visible to anyone who runs docker inspect. What is the better choice?
A build argument in the Dockerfile. A comment in the Dockerfile. A Compose secret mounted as a file under /run/secrets, read by the program. An environment variable in the Compose file.

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Five scenario questions on depends_on, health checks, job dependencies, env files and secrets. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
08/01/2026
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