CI: GKE build prunes only docker images (fix filter)

We began pruning docker images older than 6 hours old. The `docker system
prune` command also prunes volumes and networks, which isn't needed.
Furthermore, the relative-time filter was missing a negative sign.

fixes 82bb57f

Signed-off-by: Ray Bejjani <ray@isovalent.com>

test/clean-local-registry-tag.sh

@@ -21,4 +21,4 @@ docker push $1/cilium/cilium:$2
 docker push $1/cilium/cilium-dev:$2
 docker push $1/cilium/operator:$2
 
-docker system prune -f --filter "until=6h"
+docker image prune -f --all --filter "until=-6h"