Docker is an open-source project that automates the deployment of applications inside software containers, by providing an additional layer of abstraction and automation of operating-system-level virtualization on Linux. Docker uses resource isolation features of the Linux kernel such as cgroups and kernel namespaces to allow independent "containers" to run within a single Linux instance, avoiding the overhead of starting virtual machines.

The latest Docker version can be downloaded from:

Docker registries and repositories

The official public Docker registry is
The registry contains the official trusted repositories as well as user repositories.

Official repositories are jointly maintained by Docker, Inc and the application company/organization.
For example Canonical (the company behind Ubuntu) and Docker, Inc jointly maintaining the Ubuntu images.

Looking at the Ubuntu repository, this repository will hold the Ubuntu images (version 10.04, 12.04, etc).

Besides official repositories which can be trusted, there are also unofficial user repositories.
Using these user repository images can introduce serious safety implications within your application.

Docker registry and repositories