Add a Dockerfile for a PHP5.6 dev environment

[cjw6k]

I didn't have a development setup running PHP5.6 so I put one together in the
form of the php56.Dockerfile.

Build the dockerfile:
docker build -f php56.Dockerfile -t php-mf2:5.6 .

Run the container and launch bash:
docker run -it --rm php-mf2:5.6 bash

I mostly need this to run the test suite, so I added some scripts into
composer.json for phpcs and phpunit w/ code coverage. The container will run
the check-and-test-all script automatically if no other script is provided:
docker run -it --rm php-mf2:5.6

[barnabywalters]

Looks good, thanks!