diff --git a/Dockerfile b/Dockerfile
index 826fc40..ebaec27 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -8,7 +8,7 @@ MAINTAINER Adrian Malacoda <adrian.malacoda@monarch-pass.net>
 # Install ImageMagick
 RUN apt-get -y update && apt-get -y install imagemagick
 
-ENV MW_VERSION REL1_29
+ARG MW_VERSION=REL1_29
 
 WORKDIR /