Add Xdebug to the Docker container image

[sgehrig]

Feature Request

I think it might be a good idea to add Xdebug to the Docker container image (using the Dockerfile) that is distributed with the repository. This will allow to start debugging issues immediately when contributing to the library.

The enhancement is rather ease and only requires two more lines in the Dockerfile:

-   && docker-php-ext-enable mongodb
+   && docker-php-ext-enable mongodb \
+   && pecl install xdebug \
+   && docker-php-ext-enable xdebug