Enable use of TCP for docker containers

[derekoharrow]

At the moment, it appears that glances on LINUX can only use docker.sock to expose container information.

This request is to also allow the use of a DOCKER_HOST environment variable that could be set to the TCP/IP API url - e.g. tcp://localhost:2375 or tcp://localhost:2376 for HTTPS.

The rationale is that if the docker engine is restarted, containers such as glances can lose connection to the unix socket and cease receiving information, whereas the TCP/IP API will always work, even following a restart of the docker engine.

I believe this setup is already present in the Windows version of Glances, so shouldn't be hard to add to LINUX too.