Update link to Docker hub and GHCR in README.md (#629)

* Update link to Docker hub in README.md
* Add link to image page in GHCR as well

Author: candleindark

README.md

@@ -12,7 +12,7 @@ Baseimage-docker is a special [Docker](https://www.docker.com) image that is con
 
 You can use it as a base for your own Docker images.
 
-Baseimage-docker is available for pulling from [the Docker registry](https://registry.hub.docker.com/r/phusion/baseimage/)!
+Baseimage-docker is available for pulling from [the Docker registry](https://hub.docker.com/r/phusion/baseimage) and [GHCR (GitHub Container Registry)](https://github.com/phusion/baseimage-docker/pkgs/container/baseimage)!
 
 ### What are the problems with the stock Ubuntu base image?