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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: Dockerfiles That Don't Rot

Five scenario questions on the build context, .dockerignore patterns, ENTRYPOINT, CMD and stop signals. 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. A build says it is transferring 1.4 GB of context for a project whose source code is 3 MB. The Dockerfile only copies the src folder. What is the most likely cause and the right fix?
The Dockerfile has too many RUN lines. Merge them into one. Docker always sends 1.4 GB the first time and caches it forever. The build context is the whole project folder, including large folders such as test data or build output. Add them to .dockerignore. COPY src is slow, so replace it with COPY . . to speed it up.
2. Your Dockerfile has COPY . . and the project folder holds a .env file with a password. You push the image to a private registry. What is true?
The password is safe, because a private registry encrypts every file in the image. COPY skips files that start with a dot by default. Anyone who can pull the image can read the password, and deleting the file in a later step does not remove it from the earlier layer. The password is only in the image if you also print it with RUN.
3. A .dockerignore file contains the line *.txt. A file named notes/todo.txt is still copied into the image, while README.txt in the project root is not. Why?
Docker never ignores files inside subfolders. The ignore file is only read for the first COPY line. The file in notes is newer, and newer files are always kept. The pattern *.txt is matched from the context root and a star does not cross folders, so only top-level files match. Start the pattern with a double star and a slash to match at any depth.
4. An image has ENTRYPOINT ["echo", "greeting:"] and CMD ["world"]. You run docker run IMAGE everyone. What is printed?
greeting: everyone everyone greeting: world An error, because ENTRYPOINT and CMD cannot be used together.
5. A container built from your image takes ten seconds to stop and its shutdown message never appears. The Dockerfile ends with CMD python app.py; echo done. What is the cause?
Docker waits ten seconds on every stop, whatever the image does. The shell form makes sh process 1, and sh does not pass the stop signal to Python, so Docker waits and then kills the container. The echo command takes ten seconds to run. Python cannot handle stop signals.

About

Five scenario questions on the build context, .dockerignore patterns, ENTRYPOINT, CMD and stop signals. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
08/01/2026
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