Enable systemd integration

[t-8ch]

podman is able to pass systemd notification sockets through to the containerized application. By enabling the systemd integration in icinga2 the host systemd is able to see the runtime status of the containerized icinga2 instance, especially for startup notifications.

libsystemd is already installed in the base image so no new runtime dependency is introduced.

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[bobapple]

@cla-bot check

[t-8ch]

Please install it explicitly

Done

[t-8ch]

With this changeset it's possible to use the container as a Type=notify systemd service in combination with podman:

# icinga.service
[Service]
Type=notify
ExecStart=/usr/bin/podman run --name icinga --sdnotify container docker.io/icinga/icinga2

[Install]
WantedBy=default.target

This service will only be marked as ready when icinga2 from within the container has signalled its completed startup. This makes it possible to order other systemd services properly in relation to the icinga container. We want to use it to perform some post-start tasks that need the container to be properly started.