Add a couple more prerequisites

Fixed #5447

get-started/index.md

@@ -86,6 +86,15 @@ We also need to assume you are familiar with a few concepts before we continue:
 - Basic familiarity with the ideas of code dependencies and building
 - Machine resource usage terms, like CPU percentages, RAM use in bytes, etc.
 
+Finally, though we'll remind you again when you need these things, you can
+save yourself some distraction at that time by [signing up for a
+Docker ID](https://cloud.docker.com) and using it on your local machine
+by running the following command:
+
+```
+docker login
+```
+
 ## A brief explanation of containers
 
 An **image** is a lightweight, stand-alone, executable package that includes