Usually, Docker used to be "a process with cgroups and namespaces". But it has evolved since then [1], although I'd agree that fundamentally, the "process + cgroups + namespaces" is a correct answer.
It is just that nowadays the definition of a "Container" depends on the scope, and presumably "Docker" is an instance of a container. But I'd expect an interviewee to reply with "what type of OS-level virtualization tool are you referring to?", in which case you would get both a proper answer and a request for clarification :)
[1] From their own FAQ: https://docs.docker.com/engine/faq/#what-does-docker-technol...