Build the moodle-docker-wait-for-db functionality into moodle-docker-compose

[fulldecent]

Currently there is a line of boilerplate code which is necessary to communicate to everybody using this project. Is:

bin/moodle-docker-compose up -d
bin/moodle-docker-wait-for-db # <--- unnecessary boilerplate code

The experience can be improved by including that second script's functionality into the first's.

This issue is part of #304, the initiative to make it easier for anybody to get started with trying or developing on Moodle.