Switch Organizr to the new image

We at Organizr is about to push some changes that will break the container in the future, and used this opportunity to switch up the repository. It should be a drop-in-replacement (widely tested).

The new image is manifested, so no need to specify the arch. Also introduced the variables to define the branch of Organizr to pull ("master" or "dev").

Although I set the fpm variable to false, I recommend it being set to true, as it is know to somewhat increase performance. More information in the migration note