Use single mustache template to generate docker-compose.yml

Currently both `site-command` and `site-wp-command` has a seperate `docker-compose.yml`.
This makes making changes and keeping them consistent harder.

We need to keep only single `docker-compose.yml` in any one repo (preferably `easyengine` as we already need it for global `docker-compose.yml`). Whenever we need to generate docker-compose.yml, we'll use that template.