diff --git a/Dockerfile b/Dockerfile
index 052b275..1441776 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -172,7 +172,7 @@ ENV SUPERCRONIC_URL=https://github.com/aptible/supercronic/releases/download/v0.
     SUPERCRONIC=supercronic-linux-amd64 \
     SUPERCRONIC_SHA1SUM=cd48d45c4b10f3f0bfdd3a57d054cd05ac96812b
 
-RUN curl -fsSLO "$SUPERCRONIC_URL" \
+RUN curl -fsSLO "${SUPERCRONIC_URL}" \
     && echo "${SUPERCRONIC_SHA1SUM}  ${SUPERCRONIC}" | sha1sum -c - \
     && chmod +x "$SUPERCRONIC" \
     && mv "$SUPERCRONIC" "/usr/local/bin/${SUPERCRONIC}" \