So now it is possible to build the Countly Docker image simply by running:
docker build -t countly/countly-server https://github.com/Countly/countly-server.git
Countly even created an official Docker hub repository which means you do not have to build the image yourself, just run it via
docker run -d -p 80:80 countly/countly-server
The image will be pulled from the repository and launched on your host - the simplest way to try out Countly.