Add docker image and dockerized dev tools

[samizdam]

Hi guys!

I'm don't have all stuff for run php projects locally. In my mind it be helpful for contributors with docker.

[dmitrivereshchagin]

I think for most cases it's not practical to add unified development
environment to a library project. Code in a library is supposed to work
with several versions of PHP, dependencies, tools, etc. For this reason
we usually don't commit lock file in a library. I believe we should do
the same to our tools (for example we use PHP version constraint in
composer.json).

I find proposed tooling a little bit cumbersome for such a small project
which is not actively developed. As a compromise I would suggest to put
the following script in the root of the project under the name
composer and make it executable

#!/bin/sh -
exec docker run --rm -u "$(id -u):$(id -g)" -v "$PWD:/app" \
    -v composer-home:/tmp composer:1.8 composer "$@"

This script allows to perform most of the usual tasks possibly except of
code coverage reporting (depends on the PHP version).