Unknown MySQL server host 'db' Rails and Docker
While I haven't solved entirely the problem (Can't connect to the Database yet) it seems this had something to do with Nginx, and config.force_sslin production.rb
Apparently, I had a bug in the Nginx configuration. I was missing setting the X-Forwarded-Proto https header in the configuration file. This was causing infinite redirects (which I honestly don't know why they weren't present the day before... I think it was because of the cookies in my browser).
After doing that, Rails is correctly using my configuration. I still have to figure out what's the problem (which seems to be a firewall issue).