fix and mention of Symfony Docker

Author: dunglas

mercure.rst

@@ -68,17 +68,18 @@ clients.
 An official and open source (AGPL) implementation of a Hub can be downloaded
 as a static binary from `Mercure.rocks`_.
 
+If you use [Symfony Docker](https://github.com/dunglas/symfony-docker/),
+a Mercure Hub is already included and you can skip straight to the next section.
+
 On Linux and Mac, run the following command to start it:
 
 .. rst-class:: command-linux
-.. code-block:: terminal
 
     $ SERVER_NAME=:3000 MERCURE_PUBLISHER_JWT_KEY="!ChangeMe!" MERCURE_SUBSCRIBER_JWT_KEY="!ChangeMe!" ./mercure run -config Caddyfile.dev
 
 On Windows run:
 
 .. rst-class: command-windows
-.. code-block:: terminal
 
     > $env:SERVER_NAME=':3000'; $env:MERCURE_PUBLISHER_JWT_KEY='!ChangeMe!'; $env:MERCURE_SUBSCRIBER_JWT_KEY='!ChangeMe!'; .\mercure.exe run -config Caddyfile.dev