Request to provide a guide about how to use a private Nexus server to build maven project in s2i

## Potential use cases

* Download maven dependencies from a private Nexus
* Deploy jar files into a private Nexus
* Clean the local repo cache

## Potential solutions

* [Provide custom maven settings.xml file](https://github.com/kubesphere/s2i-java-container/blob/master/java/templates/README.md#custom-maven-settings)
* Provide a mirror address

See also https://github.com/kubesphere/s2i-java-container/pull/36

/area devops

Start with the custom Maven settings section in java/templates/README.md and review the referenced s2i-java-container pull request 36. Document how to use a private Nexus for dependency downloads, JAR deployment, and local cache cleanup, including the mirror-address option; done means the guide covers these listed use cases.

Written by the indexing model from the issue text.