In order to start using Yarn you will need to install Node.js, which can be installed with Node Version Manager. Referring to the GitHub page for Node Version Manager is the best way to ensure you have an up-to-date version of Node.js installed.
The following instructions are meant to be run from your host machine, not from within the docker container. This applies to all steps in this setup guide.
1. Install NVM on your host machine
NVM (Node Version Manager) is a tool that allows you to download and install Node.js. Visit this link for install instructions: Node Version Manager Installation - GitHub