Create environment variables to customize domain

Hey !

First of all, good job on the images ! Thanks for the work.

I need, for my team, to use your image for automatically testing our woocommerce plugin. But, since we use Docker for everything else in our stack, we could use your help to add some environment variables to configure the domain to use, eg: 

```
environment:
  WP_DOMAIN: http://127.0.0.1:23014
```

Thanks in advance !
