Why the process in the Docker container is running in the foreground

In the first Docker book, when the Docker container starts the Web service, it specifies the parameters that the foreground runs, such as Apache:

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1. entrypoint ["/usr/sbin/apache2"]
2. CMD ["-D", "FOREGROUND"]

Another example is Nginx:

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1. entrypoint ["/usr/sbin/nginx", "-G", "daemon off;"]

Why did you do it? Because the Docker container will remain running only when it is running in its 1th process (PID 1). If the number 1th process exits, the Docker container exits.

Why the process in the Docker container is running in the foreground