[FEATURE] DNS validation of reverse proxy configuration during config generation.

[dragon2611]

What's needed and why?

If you have a service defined that uses reverse proxy and is set to proxy to hostname. If the hostname is not resolvable for whatever reason (Container down, DNS issue/whatever) then Nginx will refuse to start/config reload because it was unable to resolve the DNS name specified in the proxy pass directive. This will either prevent new configs being applied or in case of first start may prevent any services behind bunker web from being accessible.

Implementations ideas (optional)

See if it's possible to do a DNS resolution test either with the scheduler or from the Bunkerweb container(s) prior to loading the service config, If DNS resolution fails, set the service to draft/disabled and log the reason why. (Ideally also in the UI)

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