kernels: Clean after creating an image

Remove the image and the intermediate images. Otherwise, especially
for the initial runs, the disk fills up quickly.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

scripts/kernels/centos.sh

@@ -40,5 +40,8 @@ for URL in $URLS; do
         docker build -t ${REPO}:${VERSION} -f Dockerfile.rpm --no-cache \
                --build-arg RPM_URLS="${RPM_URLS}" . &&
             DOCKER_CONTENT_TRUST=1 docker push ${REPO}:${VERSION}
+
+        docker rmi ${REPO}:${VERSION}
+        docker system prune -f
     done
 done