Sounds to me more like an overloaded website falling back to a small static error page.
1) me not understanding docker and forgetting to increase the host nofiles;
2) me not understanding upstart (because I don't normally use ubuntu...) and not realizing that it conveniently ignores /etc/security/limits.conf :)
It's obviously not a data breach: I don't actually store anyone's data :)
Sounds to me more like an overloaded website falling back to a small static error page.