CNG: minimize gitlab-container-registry
The CNG's gitlab-container-registry final image is based upon
debian:stretch-slim, while it is known that "upstream"
library/registry container is much simpler, and reduced in surface. We should look to see just how much can be stripped out of the image. Noting, that when used in Kubernetes, this image's system store (via
ca-certificates package), is actually populated via
alpine-certificates image's output.