Merge branch '3.4' into 4.3

* 3.4:
  Update XML snippet in "Define Services with a Custom Tag" section
  Add docs about mailcatcher and Docker

Author: javiereguiluz

service_container/tags.rst

@@ -243,7 +243,7 @@ For example, you may add the following transports as services:
                     <tag name="app.mail_transport"/>
                 </service>
 
-                <service class="\Swift_SendmailTransport">
+                <service id="Swift_SendmailTransport">
                     <tag name="app.mail_transport"/>
                 </service>
             </services>

setup/symfony_server.rst

@@ -305,6 +305,12 @@ MongoDB       27017 ``MONGODB_`` (set the database via a Docker ``MONGO_DATABASE
 Kafka         9092  ``KAFKA_``
 ============= ===== ======================
 
+The server also supports Mailcatcher images (including
+``schickling/mailcatcher``). When ports ``1025`` and ``1080`` are detected,
+``MAILER_*`` environment variables are added to support both Symfony Mailer and
+Swiftmailer. To access the web mailer, use ``symfony open:local:webmail`` or
+click on the "Webmail" link in the web debug toolbar.
+
 .. tip::
 
     To debug and list all exported environment variables, run ``symfony