The original FireFly documentation does help in trying to get this to work. It's of course Linux based, but I wanted to make it with Powershell.

You can find the script here: https://github.com/MoebiusZero/Scripts/tree/main/Powershell/FireFly%20Importer
The explanation on how to use it can be found there in the read me. 

Just keep in mind the script looks for the JSON fle on the computer that is running on the script, not inside the container.
I wasn't sure of this either untill I tested it the first time around.