The history is a bit fuzzy. The interesting feature introduced in 2.6.24 was PID and network namespaces. Containers were "complete" by Linux 3.8 with user names

The history is a bit fuzzy. The interesting feature introduced in 2.6.24 was PID and network namespaces. Containers were "complete" by Linux 3.8 with user namespaces. Cgroups are not that important to be able to build containers (isolation first). There were other out-of-tree technologies before that, notably VServer and OpenVZ.