diff --git a/Dockerfile b/Dockerfile
index fd7ed97..fc5ff56 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -23,7 +23,7 @@ RUN ./build-llvm.sh $TOOLCHAIN_PREFIX && \
     ./strip-llvm.sh $TOOLCHAIN_PREFIX && \
     rm -rf /build
 
-ENV TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
+ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
 
 # Install the usual $TUPLE-clang binaries
 COPY wrappers/*.sh wrappers/*.c ./wrappers/
diff --git a/Dockerfile.dev b/Dockerfile.dev
index a09fe98..7ce79fb 100644
--- a/Dockerfile.dev
+++ b/Dockerfile.dev
@@ -29,7 +29,7 @@ RUN ./build-llvm.sh $TOOLCHAIN_PREFIX
 COPY strip-llvm.sh .
 RUN ./strip-llvm.sh $TOOLCHAIN_PREFIX
 
-ENV TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
+ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
 
 # Install the usual $TUPLE-clang binaries
 COPY wrappers/*.sh wrappers/*.c ./wrappers/