Docs: Limit fail2ban matches to front container

Previously fail2ban matched against all journal entries. This patch
adds a tag to the logdriver and fail2ban filter documentation that
limits the matches to entries from the front container

docs/faq.rst

@@ -497,6 +497,8 @@ follow these steps:
 
   logging:
     driver: journald
+    options:
+      tag: mailu-front
 
 2. Add the /etc/fail2ban/filter.d/bad-auth.conf
 
@@ -506,6 +508,7 @@ follow these steps:
   [Definition]
   failregex = .* client login failed: .+ client:\ <HOST>
   ignoreregex =
+  journalmatch = CONTAINER_TAG=mailu-front
 
 3. Add the /etc/fail2ban/jail.d/bad-auth.conf