Sorry for not seeing your comment until now. Amazingly great vuln BTW. It's early in 2017, but this is probably going to be one of this year's best.
It's very important for everyone to understand my advice is RHEL/Fedora specific, which is---I think---the source of the misunderstanding here.
Putting aside `ptrace` being the best way to guarantee a race win, the reason for my focus on `CAP_SYS_PTRACE` is that with SELinux enabled there is no other way to exploit having access to the file descriptors. Even if you explicitly try to pass a containerized process an external file descriptor "legitimately" (e.g. with `sendmsg`) SELinux will still ultimately block the access due to the type restrictions. This means that with `setenforce 1` you need to use something like code injection to get the external process to access the file descriptors on your behalf.