You set this in the SystemD unit file for the Ignition service. Consider specifying AmbientCapabilities=CAP_NET_RAW CAP_NET_BIND_SERVICE to allow Ignition to use ports 80 & 443 and to use built-in ping when not root.
Make sure to change ownership of all Ignition files and folders to the new user.
I have edited the service configuration: "sudo nano /etc/systemd/system/Ignition-Gateway.service", but the "AmbientCapabilities" variable does not exist.
Should I add it like this: AmbientCapabilities=CAP_NET_BIND_SERVICE?
Also, consider using a systemd override file instead of editing the main file, e.g.:
systemctl edit Ignition-Gateway.service
This will create an override file at /etc/systemd/system/Ignition-Gateway.service.d/override.conf that will persist through an Ignition upgrade (that may reset/revert that base configuration file).
I should probably also mention that the override file is for augmenting the base file, you don't need to put the contents of the original file in there, otherwise you'll end up with conflicts there.