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/mainline.sh

@@ -51,4 +51,7 @@ for KDIR in $KDIRS; do
     fi
     build_image ${VERSION} ${KDIR} && \
         DOCKER_CONTENT_TRUST=1 docker push ${REPO}:${VERSION}
+
+    docker rmi ${REPO}:${VERSION}
+    docker system prune -f
 done