Docker is a tool designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and ship it all out as one package.

Yes and community version is Open-Source.

Docker has the unique advantage of being a portable container.  The containerization of cryptocurrency software makes it more portable and secure by only allowing execute of code inside the Docker container itself.  This method never exposes the host system to the same threats that a typical user would encounter.

Docker is inherently more secure than running programs on a host system.  Docker is a container and cannot access the host system.

No!  You can use Docker to mine cryptocurrency with CPU.

nvidia-docker is a package developed by NVIDIA for the purpose of accessing GPU hardware inside of a Docker container.  You need to install nvidia-docker if you have a NVIDIA gpu and would like to mine with Docker.

To get started you should follow the Getting Started guide.