The default docker runtime does not run the containers in a VM but instead on top of the host kernel using namespaces. In this case the kernel which the container utilizes is the same kernel running the rest of the system. The container is limited by the CPU architecture and configuration of the underlying kernel.

If your container runtime uses an emulator (say QEMU) you are able to run containers which do not match the host system, albeit at a slower speed than running the container using a matching architecture and namespaces.