Occasionally, new features and existing bugs need to be tested during install or update between Joomla! versions. The build script found in the CMS repo on GitHub can be easily modified to build packages from a feature branch, and will result in the full array of packages (full install and update) available for use.
In order to build the packages, your local checkout must be in sync with the base Joomla! CMS repo, to include all tags. This is important because the build script uses the git tags created at each release to build the update packages. Next, your feature branch should be in sync with the master branch from the Joomla! CMS repo to ensure that the current code is being tested in addition to your proposed changes.
To build packages from your feature branch, all you need to do is make two changes to the build script.
So long as you don't have any PHP syntax errors and each of the release tags are available in your local checkout, you should be able to run the build script and have full packages ready for testing.