[Symfony CLI] Document about APP_ENV=test behavior

Running the Symfony CLI with `APP_ENV=test` has an interesting but undocumented behaviour, it updates `DATABASE_*` environment variables for a test environment.

Author: Kocal

setup/symfony_server.rst

@@ -322,6 +322,22 @@ prefixed with ``DB_``, but as the ``com.symfony.server.service-prefix`` is set
 to ``DATABASE``, the web server creates environment variables starting with
 ``DATABASE_`` instead as expected by the default Symfony configuration.
 
+You don't need to create two containers for a main database and a test database.
+Using `APP_ENV=test symfony` will automatically adjust `DATABASE_*` environment variables
+for a test environment.
+
+.. code-block:: terminal
+
+    $ symfony var:export --multiline 
+    export DATABASE_DATABASE=app
+    export DATABASE_NAME=app
+    export DATABASE_URL=postgres://app:app@127.0.0.1:49160/app?sslmode=disable&charset=utf8
+
+    $ APP_ENV=test symfony var:export --multiline 
+    export DATABASE_DATABASE=app_test
+    export DATABASE_NAME=app_test
+    export DATABASE_URL=postgres://app:app@127.0.0.1:49160/app_test?sslmode=disable&charset=utf8
+
 Here is the list of supported services with their ports and default Symfony
 prefixes: