fix docker run command in README.md

[tkothe]

docker run was missing rootshell prefix when referencing the image to run.

[tkothe]

we should probably update this even more to expose the 443 port. Without that, it won't be usable from the host machine. What do you think @xme ?

[tkothe]

and on a completely unrelated note: unfortunately for me this solution is not working. I was trying to convert the emails that Uber currently sends as their invoice emails to PNG. They don't convert well with this wkhtmltoimage based solution. It basically only shows the map of the Uber ride