Why and what env_vars to set while creating a mix release?

Another opportunity to learn - what is staging?

Staging environment is a great use case for that.

My laptop is dev and VPS is prod. What is staging ?

Staging is an environment that should be exactly like production, but allow you to test a new version of the app before going to production.
As you know it is not rare for an app to work on the developer machine and to crash on production environment.

Hmm that makes sense. Dont think I need staging in the near future for my meagre needs.