Considering the RouterOS document(https://help.mikrotik.com/docs/display/ROS/Container#Container-c)buildanimageonPC) is suggesting using BuildKit to build the image, if we also use BuildKit to save the .tar file like this:
docker buildx build --no-cache --platform arm64 . --output type=docker,dest=myimage.tar
The command above will generate a .tar file directly when building (in RouterOS documentation, it uses a separated command docker save to generate the .tar file). However, RouterOS can not load myimage.tar generated in this approach. (though using docker load -i myimage.tar can load it properly in docker)