If you want to overwrite your existing authorized_keys file, replace >> with >. For an example of the json file, see [example.json](https://git.jcg.re/jcgruenhage/borgocli/src/master/generate/authorized_keys/example.json).
This is everything for now, but I hope to extend that later. This might get some functionality only useful to my docker images, since that is what I am writing this for.