Fix docker image cleanup

Makefile

@@ -41,7 +41,7 @@ osd_docker:  /opt/qemu/bin
 	if ! ls /proc/sys/fs/binfmt_misc | grep -q qemu ; then sudo ./docker/qemu-binfmt-conf.sh --qemu-path /opt/qemu/bin --persistent yes; fi
 	TAG="wfb-ng-osg:build-`date +%s`"; docker build -t $$TAG docker --build-arg SRC_IMAGE=$(DOCKER_SRC_IMAGE)  && \
 	docker run -i --rm -v $(PWD):/build $$TAG bash -c "trap 'chown -R --reference=. .' EXIT; export VERSION=$(VERSION) COMMIT=$(COMMIT) SOURCE_DATE_EPOCH=$(SOURCE_DATE_EPOCH) && cd /build && make clean && make osd mode=$(mode)"
-	docker image ls -q "wfb-ng-osd:build-*" | tail -n+5 | while read i ; do docker rmi $$i; done
+	docker image ls -q "wfb-ng-osd:build-*" | uniq | tail -n+6 | while read i ; do docker rmi -f $$i; done
 
 clean:
 	rm -f osd *.o *~