syslog: change it to unix domain socket

docker is using a extra udp port for every container. when there is
a lot of containers, a lot of random udp ports get used up. this causes
problems when installing apps that require contiguous port ranges

src/mailserver.js

@@ -179,7 +179,7 @@ async function configureMail(mailFqdn, mailDomain, serviceConfig) {
                 --net cloudron \
                 --net-alias mail \
                 --log-driver syslog \
-                --log-opt syslog-address=udp://127.0.0.1:2514 \
+                --log-opt syslog-address=unix://${paths.SYSLOG_SOCKET_FILE} \
                 --log-opt syslog-format=rfc5424 \
                 --log-opt tag=mail \
                 -m ${memory} \