text: – Control groups: A control group (cgroup) is the technology that limits an application to a 
  
  specific set of resources. This feature allows Docker Engine to share available 
  hardware resources to containers and optionally enforce limits and constraints. 
  
  – Union file system: Union file systems (UnionFS) are file systems that operate by 
  
  creating layers, which makes them lightweight and fast. Docker Engine uses UnionFS 
  to provide the building blocks for containers. 
  
  – Container format: Container format is the wrapper around NameSpaces, control 
  groups, and UnionFS. The default container format is libcontainer. 