Bundle grpahViz and use non root user

[agalazis]

By not bundling grpahviz it cannot be used as a standalone tool. What I was interested in is a docker image for a pre-commit hook. With this image, I can do the following pre-commit config:

      - id: terraform-inframap
        name: terraform-inframap
        language: docker_image
        entry: --entrypoint /bin/sh agalazis/inframap
        args: ['-c' , 'for file in $(find /src -name "*.tfstate" -o -name "*.tf") ; do output=${file%%.*};  /home/inframap/inframap generate $file | dot -Tpng  > "${output}.png" ;done;']
        pass_filenames: false

Also, using root is not ideal. So I configured a user for it.

[xescugc]

Thanks @agalazis , as this is an old PR I took the liberty to fix the ENTRYPOINT and also update a few things on the README