Docker support

I start this pull request to discuss how to merge this two branches.
The Dockerfile installs everything needed to use ionic in a docker container.
It would be possible to use docker inside vagrant to support all platforms without having to maintain two parallel systems.
The reason why to use the dockerfile instead of the bash installer is because the docker's layer functionality. If we upgrade the dockerfile, only the binary differences are uploaded to the common hub, so everyone that downloads the binary version, would benefit of small downloads instead of having to build or download the entire image each time.
Also, the docker image can run inside a vagrant instance... so perhaps we can do that.