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/debian.sh

@@ -26,4 +26,7 @@ for KERN_DEB in $KERNELS; do
     docker build -t ${REPO}:${VERSION} -f Dockerfile.deb --no-cache \
            --build-arg DEB_URLS="${URLS}" . &&
         DOCKER_CONTENT_TRUST=1 docker push ${REPO}:${VERSION}
+
+    docker rmi ${REPO}:${VERSION}
+    docker system prune -f
 done