Docker is not strictly a virtualization technology (it shares some features with it), but one of the main differences is that it shares a kernel with the host.
The image you’re looking at is intended to be used as a base image for other images, if you just run it, it will exit.
You could try using distrobox, but systemd units don’t run in that environment (it’s treated as a chroot environment), and unless you set it up to run as root, you can’t just use ‘su’ or ‘sudo’ to become root.
Alternatively, you could just use the freeradius server docker image provided by the freeradius project on Dockerhub.