Decidim is a Ruby on Rails application, and as such we try to follow their conventions. Please read Configuring Rails Applications.
Just as a reminder, these files are important:
By default, the bulletin board will store files, such as verifiable election logs, on disk. This is not advisable, and you should edit
config/storage.yml to configure a cloud storage provider such as Amazon S3. Follow instructions at https://edgeguides.rubyonrails.org/active_storage_overview.html#setup to do so.
You can change most of these settings through Environment Variables.