Update DOCKER_README.MD (#716)

The cask command no longer works, adjusting it  and there is a $ in front of some of the commands so removing these so they can be directly copied and pasted into the terminal

Author: matthewbond

DOCKER_README.MD

@@ -5,7 +5,7 @@ This guide is made for OSX users.
 
 First install Docker using [Homebrew](https://brew.sh/)
 ```
-$ brew install docker
+brew install docker
 ```
 
 You can then install [Docker Desktop](https://docs.docker.com/get-docker/) if you wish, or use `docker-machine`. As we prefer the second option, we will only document this one.
@@ -14,22 +14,22 @@ You can then install [Docker Desktop](https://docs.docker.com/get-docker/) if yo
 
 Install `docker-machine`
 ```
-$ brew install docker-machine
+brew install docker-machine
 ```
 
 Then install [VirtualBox](https://www.virtualbox.org/) with [Homebrew Cask](https://github.com/Homebrew/homebrew-cask) to get a driver for your Docker machine
 ```
-$ brew cask install virtualbox
+brew install virtualbox --cask
 ```
 
 You may need to enter your password and authorize the application in your `System Settings` > `Security & Privacy`.
 
 Create now a new machine, set it up as default and connect your shell to it (here we use zsh. The commands should anyway be displayed in each steps' output)
 
 ```
-$ docker-machine create --driver virtualbox default
-$ docker-machine env default
-$ eval "$(docker-machine env default)"
+docker-machine create --driver virtualbox default
+docker-machine env default
+eval "$(docker-machine env default)"
 ```
 
 Now you're all setup to use our provided Docker image!