Docker is a virtualization technology that be able to make virtual (server) environment easily without installing OS. If you have tried to make virtual environment before, you might think as follow, "WTF.. i just want to install Apache, and to test my server without spending my time to install OS!!", Or "I don't wanna install LAMP services on my HOST system (Laptop, Desktop), i need a virtual environment which are quick, easy, not much time for installing."
Docker will help you.
Just install and to try some examples in Docker homepage.
pull ubuntu:14.04 docker image for Xenforo on Docker hub.
To run ubuntu:14.04 image of Docker with option as -p 8080:80 (this means port binding from 8080 (HOST) to 80 (Docker image))
Then you can do anything on the virtual image, and if you wanna remove the docker image which you have tested, do docker rm <image name>
Who are used to Linux system can understand easily, and use quickly.
I spent 15 mins to build up test server, and to install LAMP services with Docker.