Symlinked /var/run/docker.sock causes problems on CircleCI

[binocarlos]

There have been reports of problems running powerstrip on CircleCI because /var/run is symlinked to /run and so when we try to mount the docker sockets using a volume - it fails because volumes don't like symlinks.

The solution is to mount /run/docker.sock rather than /var/run/docker.sock - perhaps a note about this is needed in the README.

Example docker run with symlink accounted for:

sudo docker run -d --name powerstrip \
  -v /run/docker.sock:/var/run/docker.sock \
  etc