Create a `decidim-install.sh` script that can handle the installation of a Decidim instance

[davidbeig]

We need a script that could handle the installation of a Decidim instance in a newly created server, so that a user without much technical knowledge could do this process on it's own.

Things that need to be sorted out before the script:

• Have a working pipeline to generate the Decidim images. Right now the :latest version is pointed to v0.28.6 and the latest decidim version built is the v0.29.1.
• Queue system: Sidekiq is mandatory since v0.30. That means that we need a redis-compatible cache.
• How to handle migrations. The decidim images, currently, are being generated with the decidim-generator and then pushed to the repository. That means that every image has a set of db migrations that don't share a timestamp.

This script should:

• Know what OS it is being executed on.
  • First only available for debian based OS.
• Specify decidim version (:latest, :community, v0.29, etc) and registry (ghcr.io/docker hub/etc). // ghcr.io/decidim/decidim:latest
• Check for docker installation (maybe install it)
• Guide the user on the necessary information
  • Application name
  • System admin
  • Host url
• SMTP credentials
• Maps credentials
• Check if the server is accessible. IP/domain and open ports (80 and 443)
• Create a folder that holds the docker-compose.yml and the configuration needed.
  • volume for storage
  • volume for db/migrations
• Reverse proxy (Nginx or Traefik, as to handle also the SSL certificates)

Evolution of the script:

• Handle updates
• Handle rollbacks
• Local backups of the database
• Module installations

This script should have a well-documented entry in this repository/the decidim repository and docs.decidim.org.

References:

• Discourse: Install cloud
• PokeCode: Decidim Install
• Dokploy: Installation
• Omakub