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Please tell us about dockerfileI don't understand the meaning of the last line.

I tried to explain the dockerfile.

Error message
Bring centOS from the official docker image.
Install http daemon (Apache) with yum package.
Copy index.html in the current directory to the Apache data storage location.
Open the 80 and 443 ports of the container.
** Apache httpd program, foreground? (The background is because D is on?) **
Applicable source code
FROM centos: latest
RUN yum install -y httpd
COPY ./index.html /var/www/html/index.html
EXPOSE 80 443
CMD ["/ usr/sbin/httpd", "-D" "FOREGROUND"]
I want to hear most

I don't understand the meaning of the last line, whether it is foreground or background, and what does background mean in the first place? Please kindly advise.