Huh, I missed that earlier. According to that, servers.json is a directory. It should be a file (note the d at the front of the ACL).
Ownership doesn't really matter, as long as the file is readable by UID 5050, and all the parent directories also have read and execute permissions that allow UID 5050 to use them.
Typically, /root is not accessible to users other than root - and it should stay that way; create another directory somewhere to put the servers.json file in.
I suspect the container (which runs under UID 5050) cannot read files in /root on your host. The file has permissions that would allow that, but does the directory it's in?