Provide Docker image for testing under different Java versions

[Andrey Somov]

Create dockerfile(s) to simplify testing under different Java versions

[Andrey Somov]

These are the commands to run tests under Java 8:

cd <snakeyaml>/docker
docker build --tag=snakeyaml:dev java8
docker run -it snakeyaml:dev bash
mvn test

[Alexander Maslov]

runs fine for me.

OSX 10.10.3 Docker version 1.7.0, Kitematic (Beta) 0.7.2

[Andrey Somov]

Other images can be added later, when wee need them.