Fixed example Docker command in README

[thom1337]

The Docker example in the README fails:

docker -- run -p 9999:9999 gcr.io/android-battery-historian/stable:3.0 --port 9999
docker: 'run' is not a docker command.
See 'docker --help'

When -- is removed it works fine:

docker run -p 9999:9999 gcr.io/android-battery-historian/stable:3.0 --port 9999
2019/07/15 16:14:28 Listening on port:  9999