new: [synapse] support ``server-name``.

Notice that you have to add a special DNS rule in the ``server-name`` zone::

    _matrix._tcp.example.com 3600 IN SRV 10 0 8448 matrix.example.com

This is to declare that ``matrix.example.com`` is matrix server for
domain ``example.com``.

synapse/hooks/init

@@ -19,6 +19,8 @@ options="$(e "$service_def" | shyaml -y get-value options)" || true
 
 
 SYNAPSE_OPTIONS=(
+
+    server-name:string       ## The server name
     report-stats:bool        ## Enable anon stat reporting back to the Matrix project
     enable-registration:bool ## Enable registration on the Synapse instance.
     allow-guest:bool         ## allow guest joining this server.

synapse/hooks/web_proxy-relation-joined

@@ -4,20 +4,23 @@ set -e
 
 . lib/common
 
-DOMAIN=$(relation-get domain) || exit 1
 
+env=()
 url=$(relation-get url)
-
 if [ "${url%://*}" == "https" ]; then
-    no_tls="    SYNAPSE_NO_TLS: 'yes'"
-else
-    no_tls=
+    env+=(SYNAPSE_NO_TLS "'yes'")
 fi
 
+server_name=$(options-get "server-name") || true
+if [ -z "$server_name" ]; then
+    DOMAIN=$(relation-get domain) || exit 1
+    env+=(SYNAPSE_SERVER_NAME "$DOMAIN")
+fi
+
+if [ "${#env[@]}" -gt 0 ]; then
     init-config-add "
 $SERVICE_NAME:
   environment:
-    SYNAPSE_SERVER_NAME: $DOMAIN
-$no_tls
+$(printf "    %s: %s\n" "${env[@]}")
 " || exit 1
-
+fi