diff --git a/Dockerfile b/Dockerfile
index 7d0c91d..1a74cda 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,5 @@
-FROM buildpack-deps:$release-name
+ARG release-name=unstable
+FROM buildpack-deps:${release-name}
 
 LABEL org.opencontainers.image.source=https://github.com/rust-lang/docker-rust