Improve Container Registry description

This is current container registry description:

A 'container image' is a snapshot of a container. You can host your container images with GitLab. 
To start using container images hosted on GitLab you first need to login:

docker login dev.gitlab.org:5005

Then you are free to create and upload a container image with build and push commands:
docker build -t dev.gitlab.org:5005/kamil/velociraptor/image .

docker push dev.gitlab.org:5005/kamil/velociraptor/image

It doesn’t say anything about multi-level images, and options that it brings. We should improve the wording here, maybe link to documentation, and make this information more attractive visually.

@axil @dimitrieh @markpundsack Can you help?