When syslog-ng is managed as an runit service, application logs may be
lost during shutdown, when the syslog-ng service stops before the app
exits.
This change uses the my_init.d directory to manage syslog-ng at a lower
runlevel than runit.
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
