Docker itself is pretty cool, but when you add remote control of multiple machines it gets really powerful. You can essentially use docker-machine to control remote cloud instances or to control virtual machines that are running locally.
There is also a powerful idea called generic which means you can start docker container on any linux machine where you have root and ssh access through the generic driver.
There is a slight typo because you need to provide the private not the public key, but what it does then is change the hostname of the machine and then you can use
docker-machine env new-machine and then every docker command you run actually runs on this machine.
Also if you have a bunch of machines say in a your local shop that are free, you can create a swarm and then jobs will go to those machines as they are free. pretty cool!
You get some pretty massive lines, but basically, you just have a single command line per development machine and they all work together.