Add instructions for docker

[pesterhazy]

I couldn't get it to work on macOS with the pre-installed version of ruby - it failed building a dependency called curb.

This adds instructions that allow you to run the script without worrying about deps

[Strech]

Thanks for the issue @pesterhazy.

Yeah, curb is known to be tough to build sometimes, instructions are one way to solve the issue, another way is to have a Dockerfile with a recipe. I would like to suggest that way, WDYT?