In this article, <SMARTIDHOME> refers to /home/nexus, but this can be different depending on the setup.
Open <SMARTIDHOME>/docker/compose/identitymanager/operator/docker-compose.yml for editing.
Set properties as needed, by adding/editing the SYSTEM_PROPERTIES environment variable. This variable takes a JSON as value, and can be set like this. (See List of Identity Manager system properties for available properties)