Hi… On Linux, you need superuser rights (i.e., root) to bind to port 53.
More generally… UDP is hard-mode Fly.io, so I would suggest minimizing use of privileged ports, auto-stop, etc., at first. Once you have something basic working, then progressively lock things down more and more, 
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(Maybe even try the super-simple Netcat test, at first.)
Hope this helps a little!