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/docker.js

@@ -345,7 +345,7 @@ async function createSubcontainer(app, name, cmd, options) {
                 Type: 'syslog',
                 Config: {
                     'tag': app.id,
-                    'syslog-address': 'udp://127.0.0.1:2514', // see apps.js:validatePortBindings()
+                    'syslog-address': `unix://${paths.SYSLOG_SOCKET_FILE}`,
                     'syslog-format': 'rfc5424'
                 }
             },