Well, I keep reading this, but what if you design an app that relies on local storage, for example one that uses an embedded database like SQLite, LevelDB or BoltDB? 12 factors don't address this.
Building stateless containers is good practice.
There will always be exceptions but it's good to know the tradeoffs before making those exceptions.
12 Factors only address stateless processes, which is the easy part. But most applications are stateful. It doesn't matter if data are stored in an embedded database (like SQLite, LevelDB, BoltDB) or in a client-server database (like PostgreSQL, MySQL, MongoDB, RabbitMQ, Redis). 12 Factors doesn't address this issue at all.