minor #8974 Add environment variables (ProfM2, javiereguiluz)

This PR was merged into the 4.0 branch.

Discussion
----------

Add environment variables

Add server environment variables, specifically for production use.

Commits
-------

ab2c135 Added the env vars in the nginx config too
45c2939 Commented the env vars and mentioned that they are optional
f64bc6b Add environment variables

Author: javiereguiluz

setup/web_server_configuration.rst

@@ -121,6 +121,11 @@ and increase web server performance:
         </Directory>
         ErrorLog /var/log/apache2/project_error.log
         CustomLog /var/log/apache2/project_access.log combined
+        
+        # optionally set the value of the environment variables used in the application
+        #SetEnv APP_ENV prod
+        #SetEnv APP_SECRET <app-secret-id>
+        #SetEnv DATABASE_URL "mysql://db_user:db_pass@host:3306/db_name"
     </VirtualHost>
 
 .. tip::
@@ -293,6 +298,12 @@ The **minimum configuration** to get your application running under Nginx is:
             fastcgi_pass unix:/var/run/php7.1-fpm.sock;
             fastcgi_split_path_info ^(.+\.php)(/.*)$;
             include fastcgi_params;
+
+            # optionally set the value of the environment variables used in the application
+            # fastcgi_param APP_ENV prod;
+            # fastcgi_param APP_SECRET <app-secret-id>;
+            # fastcgi_param DATABASE_URL "mysql://db_user:db_pass@host:3306/db_name";
+
             # When you are using symlinks to link the document root to the
             # current version of your application, you should pass the real
             # application path instead of the path to the symlink to PHP