automatically detect client host endpoint

[Lohann]

What did you implement:

Automatically detect the client host endpoint, the same way as docker-cli does.

I've implemented this feature in a project I'm working, so I decided to open a PR here in case you think this could be useful.

Steps

1. Try to load the endpoint from the DOCKER_CONTEXT environment variable
2. Try to load the endpoint from the DOCKER_HOST environment variable
3. Try to load the endpoint from the config.json file
4. otherwise use the default endpoint

[Lohann]

@vv9k