I am not sure I understand your question. Are you talking about externalizing the data directory? Then you can use the same configuration as in 2.10.x. (The documentation for 3.0.x is not finished for that)
From that link, the resolution order is:
- Java environment variable (ie. -Dgeonetwork.dir=/a/data/dir)
- Servlet context parameter (ie. web.xml)
- Config.xml appHandler parameter (ie.config.xml)
- System environment variable (ie. geonetwork_dir=/a/data/dir). ”.” is not supported in env variables
If you are looking on changes on the database configuration (or any other spring file), then you cannot use the default war installation, you should configure it on the source code and then build the war.
The war and the installer are not for complex production use, they are provided for users who just want to do quick deployments or "play" with GeoNetwork to see if they like it.
Is that what you were looking for?