It seems that grouperInstaller.jar does not use proxy settings when passed as cli args or environmental variables when it attempts to connect to I2 for downloads. tcpdump confirms grouperInstaller attempts to connect directly to software.internet2.edu over 443 when attempting to run patches.

Issue can be observed in a grouper container in an environment with an outbound proxy via:
{{cd /opt/grouper/2.3.0
java -Dhttp.proxyHost=proxy.is.here -Dhttp.proxyPort=9999 -Dhttps.proxyHost=proxy.is.here -Dhttp.proxyPort=9999 -jar grouperInstaller.jar}}

This would be a good feature to have for patching production services as well as general development in environments with enforced outbound proxies.