Docker Interview Questions and Answers

An image is a read-only template; a container is a running instance of that image.

• Image: built from a Dockerfile, composed of immutable layers, and stored in a registry. It contains the filesystem and metadata such as entrypoint, environment, and exposed ports.
• Container: an image plus a writable layer and runtime state, including processes, network, and mounts. You can run many containers from one image.

docker build -t web:1.0 .
docker run -d -p 8080:80 --name web web:1.0
docker ps

Think of the image as a class and the container as an object. Changes made inside a container's writable layer are lost when it is removed unless you use a volume. Rebuild and version images rather than editing running containers, and keep images small for faster pulls.

Both define what runs when a container starts, but they behave differently.

• ENTRYPOINT sets the executable. It is hard to override and defines the container's purpose.
• CMD provides default arguments to ENTRYPOINT, or a default command if no ENTRYPOINT exists. It is easily overridden by arguments passed to docker run.

ENTRYPOINT ["python", "app.py"]
CMD ["--port", "8080"]

Running docker run image --port 9000 replaces the CMD arguments but keeps the ENTRYPOINT. Use the exec form, the JSON array, so signals such as SIGTERM reach your process directly, which matters for graceful shutdown. Avoid the shell form because it wraps the command in /bin/sh -c and can swallow signals. Combine both for a sensible default that users can still customise.