This alpha feature has been deprecated in branch-2 and removed from trunk Please see https://issues.apache.org/jira/browse/YARN-5388
Docker (https://www.docker.io/) is, increasingly, a very popular container technology.
In context of YARN, the support for Docker will provide a very elegant solution to allow applications to package their software into a Docker container (entire Linux file system incl. custom versions of perl, python etc.) and use it as a blueprint to launch all their YARN containers with requisite software environment. This provides both consistency (all YARN containers will have the same software environment) and isolation (no interference with whatever is installed on the physical machine).