Elixir Phoenix Configuration

Set Build Pack to nixpacks
Set MIX_ENV to prod
- It should be a build time environment variable
Set SECRET_KEY_BASE to a random string (https://hexdocs.pm/phoenix/deployment.html#handling-of-your-application-secrets)
- It should be a build time environment variable
Set DATABASE_URL to your database connection string
- It should be a build time environment variable
Set Ports Exposes to 4000 (default)