synctree/sinatra-echo image provides a simple HTTP app that echoes back
the headers and parameters sent with a request along with standard CGI variables.
It is useful for debugging the HTTP payload seen by an application running behind
one or more layers of proxying.
To run the HTTP echo service exposed directly on port 80:
$ docker run -d -p 80:9292 synctree/sinatra-echo
To run the HTTP echo service behind
$ docker run -d -e VIRTUAL_HOST=echo.example.com synctree/sinatra-echo