By leveraging the templating mechanism in `settings.json`, this change allows a
Docker client to run a prebuilt image and change some basic configuration
settings, like the instance name or, more importantly, the database
coordinates.
By default, the image runs witho no administrative user enabled. If a value is
given to ADMIN_PASSWORD, the `admin` user will be activated.
Also closes https://github.com/ether/etherpad-lite/issues/3623
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Modified by muxator to support conditional user activation at runtime.
