Add example demonstrating how to build and load the extension for the official postgres docker image

[Valefant]

Hi, recently I had the use case that I needed to use the extension within the official postgres docker image. As there is no adhoc way to load external extensions in a container, I thought of creating a small example demonstrating how to build and load this extension in a dockerized environment.

@ChenHuajun

[ChenHuajun]

Well done, thank you very much.