This is a minimal repro case for the ERROR stderr to vsock zero copy err: Broken pipe (os error 32) issue we've been seeing in Fly Logs. Simply deploy this app with fly launch, and Fly Logs will begin returning the Broken Pipe error for every log line.
To fix the broken pipe, comment out these two lines in nginx.conf and redeploy:
error_log /dev/stdout info;
access_log /dev/stdout main;
With those disabled, nginx falls back to the default logging behaviour, which uses a symlink to stdout and stderr for posting logs.