To my knowledge the very first appearance was the "ctx" kernel patches project by linux-vserver.org by Herbert Pötzl et al. https://web.archive.org/web/20250307

To my knowledge the very first appearance was the "ctx" kernel patches project by linux-vserver.org by Herbert Pötzl et al. https://web.archive.org/web/20250307173707/http://linux-vserver.org/Welcome_to_Linux-VServer.org https://web.archive.org/web/20250307173707/http://linux-vser... (sadly, the domain seems to be expired just recently) I used it heavily back in the days, long before kernel namespaces went upstream and docker became a thing.