diff --git a/example/Makefile b/example/Makefile
index 9421682..c66143f 100644
--- a/example/Makefile
+++ b/example/Makefile
@@ -4,9 +4,10 @@ PLUGINS := test/plugins
 
 DOCKER = docker run -ti --rm \
 	-v $(CURDIR):/testbed \
+	$(if $(TEMPDIR),-v $(TEMPDIR):$(TEMPDIR) \
 	-v $(CURDIR)/$(PLUGINS):/home/testbed/.vim/plugins "$(IMAGE)"
 
-test: TEMPDIR=$(shell mktemp -d)
+test: TEMPDIR:=$(shell mktemp -d)
 test: test-setup
 	docker build -f Dockerfile.tests -t "$(IMAGE)" .
 	set -ex; \