Per https://github.com/mautic/mautic/issues/1793 :
Mautic depends on the local filesystem for both it’s configuration and for managing assets. This means it’s not suitable for running in Heroku or many Cloud Foundry environments because in those kinds of environments, the local filesystem is ephemeral and not shared between instances.
Mautic should be configurable to use the DB and S3 for its own config and asset storage locations, respectively. This option will make it possible to run Mautic in more places, and potentially even make Mautic deployable with a single click!