Create a new branch for next version

[ne20002]

Based on the findings of the last update of the docker images I suggest we create a second branch meant for the next release version. This is based on the assumption that the docker library builds do not rely on the branch but on the commit numbers as in the library/friendica file,

As a small project with limited resources we are likely not able to test all image version when we do an update. The update.sh script is applying updated versions or changes to the configuration defaults to all images currently existing, incuding those considered stable/productive. I handled this by only updating the dev/rc version when doing updates to the official images docker library library/friendica file. But this lacks the ability to update only version upgrades of dependencies (as all updates made in the update.sh are always applied) on stable versions.

If we introduce a new branch named 'next' we would be able to seperate the updates for the stable versions from those that are made during the development phase of the next version. And as the dev/rc version are agreed to be dev/rc/test-versions I think that this is ok for the testers. But for the stable versions I'd like to have a more stable approach of what is updated. I don't want to cause problems with running productive versions.

In fact, using two branches will require a bit more attention when updating the library/friendica file to not mix up the stable and next version changes. But I believe this is manageable.

@nupplaphil What do you say?