Honor DOCKER_HOST in the probes

[2opremio]

Right now we hardcode the docker endpoint to unix:///var/run/docker.sock

[rade]

#1626 is related.

[rndstr]

@2opremio regarding your comment:

However, this won't be the usecase since scope also needs to live in the machine being monitored (it needs /proc and conntrack for instance)

Does it even make sense to honor $DOCKER_HOST automatically? Will this lead to confusion if $DOCKER_HOST is unexpectedly set and /proc and conntrack are not from the same host? I could think of an explicit -probe.docker.env=true to alleviate this in case that's an issue, otherwise we could just go with auto-detection. Wdyt?