This weekend I did a fresh reinstall on my Rpi, using Raspbian Stretch and Portainer containers. Works like a charm. I am happy to stop using updater.py to get the latest bits and instead just recreate the running container from the latest commits.
While I have no current plans to move off the master branch, I am nonetheless curious how I would cause my container to run a non-master branch such as a beta build. I assume it would involve editing the command script which creates the container.