shopify-app-cli .env variables

[me_cookie]

Hi, I'm coming to Shopify from Laravel.  I really dig shopify-app-cli.  Installing it, and creating my first project, was a breeze on ubuntu 18.04.

Question, for testing, I would like to add environment variables for a remote MySQL database.  Are these variables out-of-the-box or do I create them myself?

Another question, will app users be able to access database credentials in such a scenario or does the "Walled Garden" hide that?  

PS my app is nodejs and react.