Merge: opportunity: official docker image

Now that nitcorn can listen on any interface, why not having lightweight standalone web applications?

How to build?
1. install docker
2. cd nit/contrib/opportunity/docker
3. docker build --tag opportunity .

How to run?
1. docker run -d -p 8080:8080 --name  opportunity opportunity
2. xdg-open http://localhost:8080
3. docker stop opportunity

or just get it from the [hub](https://hub.docker.com/r/nitlang/opportunity/tags/)
1. docker run -d -p 8080:8080 --name opportunity nitlang/opportunity

Pull-Request: #2284
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean-Philippe Caissy <jpcaissy@piji.ca>