update release 2.6.0 with CDAP docker build info for developers

The main issue is it's impossible to build from a specific branch when you give Docker a URI. If you want to build from your branch, you need to do it from a checkout, with the correct branch checked out. These instructions will always build develop. To build a 2.6-tracking container, you would need to "cd cdap" and "git checkout release/2.6.0" before running "docker build -t caskdata/cdap-standalone ."