relay/app.json

{
    "dokku": {
        "volumes": [
            {
                "host": "/var/lib/dokku/data/storage/$APP/files",
                "app": "/app/files",
                "phases": "deploy,run"
            }
        ]
    },
    "scripts": {
        "dokku": {
            "predeploy": "./bin/pre-deploy"
        }
    }
}