Support running behind reverse proxy

[clue]

Look into what is required so that this images works properly behind a reverse proxy that rewrites the paths.

For example this one:

$ docker run -d --name h5ai clue/h5ai
$ docker run -d -p 8080:80 --link h5ai:h5ai clue/frontroute

And then access http://localhost:8080/h5ai/