Dynamic $baseURL for multiple hosts

I am developing an app that will have multiple domains and subdomains pointed at it.  I would like to have CodeIgniter automatically detect the Base URL.

In Config/App.php it says this:

PHP Code:

    /*
    |--------------------------------------------------------------------------
    | Base Site URL
    |--------------------------------------------------------------------------
    |
    | URL to your CodeIgniter root. Typically this will be your base URL,
    | WITH a trailing slash:
    |
    |    http://example.com/
    |
    | If this is not set then CodeIgniter will try guess the protocol, domain
    | and path to your installation. However, you should always configure this
    | explicitly and never rely on auto-guessing, especially in production
    | environments.
    |
    */ 


I also have this line commented out in my .env file:

Code:

# app.baseURL = ''

I am receiving this error:

You have an empty or invalid base URL. The baseURL value must be set in Config\App.php, or through the .env file.

How can I dynamically set the Base Site URL to use for multiple sites pointing to the app?

My baseURL is set in the .ENV file, which if I remember rightly, takes precedence - worth checking that in the project root folder.