Add Docker support

[Javier Ferrer González]

Add a Dockerfile allowing someone to clone the repository and start developing in PHP without having to install PHP in its local machine.

• Add Dockerfile for PHP 7.2
• Add composer.phar executable
• Update README.md step by step guide on how to start using the project cloning the repo specifying how to run the tests with Docker
• Upgrade minor version in .semver

Note: We will continue to support using this project with composer create-project codelytv/php-bootstrap your-project-name because of the easiness to do so for the ones with an already local installation of PHP & Composer available. So the README.md section on how to start with Composer shouldn't be modified.

Feel free to contribute to this project solving this issue!

[Carlos Gándara]

Friendly reminder: there is a PR open since some time addressing this issue :D