Add support for a separate dockerized API container

[hopeseekr]

Building on Issue #7 (Separate vhost for APIs), create the necessary Docker plumbing so that a completely separate Dockerized PHP-FPM and Nginx container will run an API, or any separate-but-in-communication vhost.

This will in effect create the equivalent of Docker container VLANs.

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I've done this last year for a commercial project. It shouldn't be that hard for me to remember how to do it...