sd_notify support for podman to signal readiness

[ingobecker]

The images are compiled without USE_SYSTEMD=yes. While this makes sense for docker and k8s usecases having the image build with systemd support is benifitial if running them with podman. Podman allows users to run containers as systemd services which is quite a common way to deploy containers with podman. If done so, a user can instruct podman to hand the host systems NOTIFY_SOCKET down to the container so the process of the container can properly signal that it is ready to accept connections. For the moment, this is the only native way to implement a readiness probe in podman containers.

To add support for running redis containers in production with podman it would be great if it could be build with systemd support.

[tianon]

Any idea what else changes if we compile with USE_SYSTEMD=yes? Is there a size difference? Functionality difference? :eyes: