This will help the garbage collector to choose which images to keep.
The idea is that if we exclude the upstream images from the gc,
we will eventually end up in a state where we don't have to download
every now and then those upstream images, and only the forks images
would be deleted.

We can also use this as a way to clean up the registry itself:
if an image in a fork is not the upstream official one, we should
probably delete it instead of keeping millions of rebuilds. Which
means that next time the fork re-run the CI, it will use the
upstream image, which will likely already be on the runner.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>