The alternative container runtime by CoreOS in 2014.
Integrating it to Kubernetes give rise to the OCI spec. It was archived in 2019-08.
CNCF Archives the rkt Project - Cloud Native Computing Foundation
rkt, a security-minded, standards-based container engine
coreos/rkt: rkt is an App Container runtime for Linux
rkt Documentation
CoreOS is building a container runtime, rkt
App Container and Docker
What is Rocket and How It's Different Than Docker - CenturyLink Cloud Developer Center
Docker or Rocket for containers? Why not both?
Get Started with rkt Containers in Three Minutes
How CoreOS’s rkt Blazes a Secure Trail for appc Containers - The New Stack