Add git to docker container

Install git in the docker container, proposed by #203. While it is possible that issue requires further discussion, this is just a proposal pull request. If that proposal is accepted, here's the pull request.

Dockerfile

 FROM node:8.15.0
 
-# Install VS Code's deps. These are the only two it seems we need.
+# Install VS Code's deps. These are the only two it seems we need. Install git for source control.
 RUN apt-get update && apt-get install -y \
 	libxkbfile-dev \
-	libsecret-1-dev
+	libsecret-1-dev \
+	git
 
 # Ensure latest yarn.
 RUN npm install -g yarn@1.13