You can install Decidim like a regular Ruby on Rails application, with nginx, Passenger, rbenv and PostgreSQL. You should also configure a valid SMTP account to send emails and add the delayed_job gem and execute it so you can receive emails (ie on user registration).
You can follow an opinionated Rails generator to configure your decidim app so that it can be deployed to Heroku.
You can also deploy a Decidim app using Docker on hosting services that support it like Dokku, Heroku and all the cloud providers.
See the Decidim on Docker doc for more details.