to prevent running all the db engines when php is running

link postgres service to php service

Support selecting your own containers in the docker compose command.

Add support for postgres

Major Updates

- upgrade docker compose to v2
- build images locally instead of pulling them from the registry
- separate php container form nginx container
- support all the php versions including php 7.0
- remove beanstalked container to be optionally added later by the user

use git submodule instead of git clone when install

adding the different PHP versions to the docs in addition to general
updates

adding `phpnginx` or `php55nginx` will pull PHP 5.5 while `php56nginx`
will pull PHP 5.6

to avoid any errors while experimenting on the master branch

Update README.md (a typo)