Issue 2 customisable port

[exhuma]

Proposal for an entry-point that allows changing the port that is used inside the container.

This is slightly in conflict with the EXPOSE line from the Dockerfile. But it works nonetheless. For most people, port 80 works so that should not play a big role. I don't know of a way to make the EXPOSE line variable for new container start-ups as the EXPOSE line is used during the build phase.

An alternative solution would be to simply start it up on a non-privileged port by default. But that is non backwards-compatible and will break existing deployments.

I had to make a slight adjustment to the pipenv call to make it work with newer versions of pipenv

[exhuma]

I also exposed HTTPBIN_HOST but, when writing the docs I realised that this isn't really necessary. I kept it for consistency. But it might be confusing. I can remove that if you like.

[adigaboy]

Is there an update? looking forward for this change