It would be cool to facilitate testing extensions

The main case is: an extension developer is developing some new features/bug-fixes, and he wants to test those changes or let someone test them. Those changes will likely be on a branch/pull-request on its GitHub repository.

The current setup for that is to enter its already-running extension container, clone the repo, checkout the branch and do what it needs to do there to make it run. It's very messy.

It would help the developers a lot if we could improve that.

@voorloopnul suggested a simple and effective solution: allow specifying a GitHub repository + branch name in the extension's manager.

From that, Kraken would clone the repo, build the Dockerfile there and run it.

It was mentioned that the building time could be long as it's being built on a Raspberry-like computer, but it's probably an acceptable downside as it makes both users and developer's life easier, and also because most extensions are probably going to be some Python scripts + a frontend, which will probably not take more time than what it takes today to apply an update on the BlueOS version.