Consider custom port in Dockerfile healthcheck

[EugeneOne1]

Prerequisites

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Description

In #3290 we've implemented the Docker-provided healthcheck. Currently it supports parsing the configuration file, fetching the root endpoint of web UI, and requesting DNS. It also detects if AdGuard Home isn't yet configured and tries to reach the installation endpoint in this case.

Though, the official Dockerfile intends using port 3000 for installation web UI endpoint, there is a use case where user can specify some custom port via command-line options. Note also, that custom port can be specified after installation as well. Both these cases should be covered.

[EugeneOne1]

As of v0.108.0-a.603+f7dd8325, AdGuard Home no longer supports the Dockerfile healthcheck mechanism, so this issue doesn't seem to be relevant.