Documentation on docker sysctl image

[arlake228]

We should update http://docs.perfsonar.net/install_docker.html with info on the sysctl image. I would suggest something like the following:

1. Create a section at the bottom called "Running the sysctl image" or similar
2. In the section discuss what the image is, when it would be good to use and how to run it with docker-compose (you can do something similar to the docker-compose section right above it).

The docs are managed in this repo and the file that needs to be edited is here. They are all .rst file that you can convert to html using Sphinx with the "make html" target. You can see the README for more info.