Is the root user in a Docker image/container the same as the root user of the machine? Is the root user in a Docker image/container the same as the root user of the machine? I am curious about the security implications of using the root user in the Docker image.

TMK, the container is run by a non-root user (often the user is called "docker") and that user launched the docker-dameon. And the docker daemon launches the container process.
No the root in container is a root-user of container. Remember containers runs isolated.
DOcker uses cgroups and libvirt of kernel linux.