Ok, according to systemd-nspawn's manual there is an alternative to manually installing using debootstrap or similar:
Example 1. Download a Fedora image and start a shell in it
# machinectl pull-raw --verify=no \
https://download.fedoraproject.org/pub/fedora/linux/releases/32/Cloud/x86_64/images/Fedora-Cloud-Base-32-1.6.x86_64.raw.xz \
Fedora-Cloud-Base-32-1.6.x86-64
# systemd-nspawn -M Fedora-Cloud-Base-32-1.6.x86-64
This example can easily adapted for Centos. In fact they provide download support for raw images at the following link: https://cloud.centos.org/centos/8/x86_64/images/.
I hope this will help.