Add support for liveness and readiness probes

[martin-helmich]

This was requested somewhere in #209.

For a full-fledged Kubernetes deployment, the image should support liveness and readiness probes.

Some constraints that I can think of:

• When performing the installation automatically (tracked in Support for automatic installation and configuration via environment variables #230), the readiness probe should not return a successful status code until the actual setup has been completed. It should however be discussed if the liveness probe should complete successfully during installation, as to prevent a Pod from being terminated during (potentially) long-running setup phases (might also be circumvented by using startupProbes¹)
• The readiness and liveness probe should not be dependent on (mis)configurations of the CMS itself; for example, a missing root page might result in the / URL to return a 404 Not Found status code (as much as a programming error in an extension in a 500 Internal Server Error). All these issues are unrelated to the actual container health (and will not be resolved by killing and restarting a Pod) and should not be reflected in a readiness or liveness probe.

Footnotes

1. https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-startup-probes ↩