This is a simple server that responds with the http headers it received.
Image versions >= 1.4 removes the redirect introduced in 1.3.
Image versions >= 1.3 redirect requests on :80 with
X-Forwarded-Proto: http to :443.
Image versions > 1.0 run an nginx server, and implement the echoserver using lua in the nginx config.
Image versions <= 1.0 run a python http server instead of nginx, and don't redirect any requests.