Run under xvfb if $DISPLAY is not set
Removed patch that is no longer needed from Dockerfile
Dockerfile cleanup
Create plugins directory
Document Docker image

Some documentation for the Docker image in accordance with
[the guidelines for official Docker repositories](https://docs.docker.com/docker-hub/official_repos/).
Renamed logo to logo.png, as required by Docker Hub.

I think this file was vestigial, but it was the right size for
Docker Hub, so I renamed it so that it can be used.
Dockerfile cleanup to meet Docker Hub guidelines.