Docker machine is a slick tool to let you bring up a docker instance on your machine, however, it defaults to latest for the Docker version. I needed a way to test our Artifactory server with issues related to docker version, so I wanted to build a docker 1.9 instance.
Turns out this is pretty easy if you have the right urls to pass. Here's the command I used:
docker-machine create -d virtualbox --virtualbox-boot2docker-url=https://github.com/boot2docker/boot2docker/releases/download/v1.9.1/boot2docker.iso old