In order to manually install any Service Pack or Cumulative Update to SQL on Server Core all you need to do is download and run the exe with the following parameters:
/q /action=patch /allinstances /IAcceptSQLServerLicenseTerms
Example:
C:TempSQLServer2016-KB3194717-x64.exe /q /action=patch /allinstances /IAcceptSQLServerLicenseTerms
or
C:TempSQLServer2016SP1-KB3182545-x64-ENU.exe /q /action=patch /allinstances /IAcceptSQLServerLicenseTerms
Very helpful thanks!