What operating system is docker?

  docker, question

After looking at the following figure, I feel that docker is a process running on the host operating system, so after MySQL is installed directly in docker, what operating system are these software based on? Is it a host-based system? However, it seems that a redis image is installed casually on windows, and it is found that it is not the host windows operating system.
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Question 1: What operating system is docker itself? If you download redis images of Dockers one by one, it seems that you can also execute some linux commands after entering it.
Question 2: After the docker is installed with a centos operating system, for example, a Docker process is running on the host system, and centos system is running inside. Is this correct?

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Answer 1: docker is not an operating system. Docker is a container, which can be compared to virtual hardware. The operating system is installed on the hardware (host). The mirror image of redis is definitely a system with redis installed, not just a redis. You can enter the mirror image and use relevant commands to view system information.
Answer 2: It is almost understandable that docker is a container. As for whether docker is a single process, you can check more detailed information or check official documents and source codes.