Prod packaging v1 (#23)

* Sets media directory for prod

* Improved logging; Added docker files for prod

* Improved fetching SECRET_KEY_BASE for selfhosted instances

rel/overlays/bin/docker_start

@@ -0,0 +1,5 @@
+#!/bin/sh
+/app/bin/migrate
+
+cd -P -- "$(dirname -- "$0")"
+PHX_SERVER=true exec ./pinchflat start