Adding the possibility to use Dev Container for building

[dhobi]

Motivation

As stated in #59 it would be nice to have an easy/integrated way to setup a local WordPress instance. Beside WordPress, there is also the need to manually check and/or install the xmllint and xsltproc dependencies

Solution

This PR adds the possibility to use a Dev Container for development by using a DIND (docker-in-docker) container with all the dependencies already installed.
For the first installation of the WordPress instance, a detailed instruction has been added to the README.md

The setup should be as easy as

1. Open this repository in Visual Studio Code with Dev Containers extension installed
2. Once the Dev Container has started up: docker-compose up
3. Complete Wordpress installation on: http://localhost:8080