Make use of Dockerfile HEALTHCHECK feature

[Al2Klimov]

so that Compose can tell whether we actually started successfully.

[Al2Klimov]

https://docs.docker.com/engine/reference/builder/#healthcheck

[Al2Klimov]

Idea

• WIP: /var/run/icinga2/icinga2.s: reload on POST /v1/reload icinga2#7349

The same socket can be checked via a HEALTHCHECK curl whether Icinga is still alive. The latter info can be drawn from our own watchdog updates.