As you can see from your volume mappings, only a subfolder of /nobody is actually mapped to unRAID. So, /nobody itself is inside the container, and the file you have created is taking up space in your docker .img file.
You should remove the container and image in order to clean up your docker .img file. Then add the container again from your template, and when you use it, make sure you make it output to a folder that is mapped to unRAID. That is probably what your /Output folder is for.
if he wants the converted file outputted, if he hasn't killed the container yet that is...
docker exec -it HandBrake mv /nobody/*.m4v /Output/
from the unraid command line should give him it.
Wasn't sure I should give him that opportunity to pollute his docker .img further since his understanding is a little uncertain.