Security of Docker as it runs as root user

Security of Docker as it runs as root user

A Docker blog post indicates:
Docker containers are, by default, quite secure; especially if you take
care of running your processes inside the containers as non-privileged
users (i.e. non root)."
So, what is the security issue if I'm running as a root under the docker?
I mean, it is quite secure if I take care of my processes as
non-privileged users, so, how can I be harmful to host in a container as a
root user? I'm just asking it to understand it, how can it be isolated if
it is not secure when running as root? Which system calls can expose the
host system then?