Yep, that's exactly what happened. Combination of: 1) me not understanding docker and forgetting to increase the host nofiles; 2) me not understanding upstart

Yep, that's exactly what happened. Combination of: 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 :)