Adds support for running WebRisk Proxy Server as a container

[pantuza]

Hi, folks.

This Pull Request tries to contribute adding the capability of running the WebRisk Proxy server
as a Docker container. It has the following contributions:

• Creates a Dockerfile which instructs how to build a docker image with the WebRisk Proxy Server;
• Adds documentation on the project Readme explaining how to build and run a container;

This docker image allows two runtime environment variables:

• WR_API_KEY: For informing at container runtime/load the Google API key to be used by the WR Proxy server;
• WR_PROXY_PORT: For allowing the user to modify which port the container is going to bind the WR Proxy server;

I believe this contribution can help new comers or new users of the WebRisk API to get started
and try it in their environments.

---

If you folks think this contribution makes sense to this project, I would suggest to automatically build
this docker image in a regular and automated basis and make it available at Google DockerHub namespace.

With that, users could easily try the WebRisk Proxy Server by simply running:

$> docker run -p 8080:8080 -e WR_API_KEY=XXX google/webrisk

All the best, and thank you for this project.