Require APP_KEY to be defined in environment

Summary:
The Cachet documentation stresses APP_KEY should be stable and not volatile, as
the lack of key would "result in all of your encrypted/hashed data being lost".

Reference: https://docs.cachethq.io/docs/installing-cachet

Test Plan: Run container with and without APP_KEY set

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2256