Improve Docker config injected in Core's container



The docker socket path injected in the Core’s container is a constant and only works on Linux system.
API Version, TLS certificates are also not injected in the Core’s container.


Fetch the docker config from ENV variables that Docker expose on the host machine, then pass them to the Core’s container on start.

Reply from @Nicolas

I think we should improve the Docker’s config anyway. It’s never good to have config in constant.
We should read the local Docker config.

Reply from @Anthony

This would be useful also if we want to connect to the core through docker-machine

The engine is now using the docker client ( that will read from the env and if no env are provided will fallback on the correct socket based on the OS