Add syslog & fix /data/git rights & SOCAT_LINK env var

- Resolve #1893
- Add syslogd to output sshd log on stdout (via `docker logs`)
- Enforce directory rights on `/data/git`, `/data/gogs` & `/data/ssh`
- Add `SOCAT_LINK` environment variable to prevent the creation of scout links when they are not needed (see #1815)

docker/s6/syslogd/run

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+if test -f ./setup; then
+    source ./setup
+fi
+
+exec gosu root /sbin/syslogd -nS -O-