How is the first process inside a Docker container started?

When a Docker container is started, the first process inside it is the process specified in the CMD or ENTRYPOINT instruction in the Dockerfile.

This process runs with PID 1 inside the container and is responsible for executing the main task of the container.

For example, if the Dockerfile specifies:

ENTRYPOINT ["python", "app.py"]

then the process python app.py will be started when the container runs.

If not explicitly specified, Docker uses the default command from the base image.

It is important that this process correctly handles signals and does not exit immediately, otherwise the container will stop.