Made services installation optional during build

You can user `ENV` directive in Dockerfile to disable the installation
for some services or change `image/buildconfig`.

The flags are :

DISABLE_SSHD
DISABLE_CRON
DISABLE_SYSLOG

image/services/syslog-ng/syslog-ng.runit

@@ -0,0 +1,32 @@
+#!/bin/sh
+set -e
+
+# If /dev/log is either a named pipe or it was placed there accidentally,
+# e.g. because of the issue documented at https://github.com/phusion/baseimage-docker/pull/25,
+# then we remove it.
+if [ ! -S /dev/log ]; then rm -f /dev/log; fi
+if [ ! -S /var/lib/syslog-ng/syslog-ng.ctl ]; then rm -f /var/lib/syslog-ng/syslog-ng.ctl; fi
+
+SYSLOGNG_OPTS=""
+
+[ -r /etc/default/syslog-ng ] && . /etc/default/syslog-ng
+
+case "x$CONSOLE_LOG_LEVEL" in
+  x[1-8])
+    dmesg -n $CONSOLE_LOG_LEVEL
+    ;;
+  x)
+    ;;
+  *)
+    echo "CONSOLE_LOG_LEVEL is of unaccepted value."
+    ;;
+esac
+
+if [ ! -e /dev/xconsole ]
+then
+  mknod -m 640 /dev/xconsole p
+  chown root:adm /dev/xconsole
+  [ -x /sbin/restorecon ] && /sbin/restorecon $XCONSOLE
+fi
+
+exec syslog-ng -F -p /var/run/syslog-ng.pid $SYSLOGNG_OPTS