Rkt is the Container runtime from CoreOS based on the APPC specification. The following are some differences in Rkt compared to Docker:
There are three stages in the Rkt execution:
Stage0: This does the image discovery and retrieval and sets up a filesystem for stages 1 and 2.
Stage1: This sets up the execution environment for the container execution using the filesystem set up by
stage0. Rkt uses systemd-nspawn to set up cgroups, networking, and so on in this stage. The goal here is to keep
stage1swappable by other implementations. ...