docker: fix a failure when building crosvm

What fixed by this change failed crosvm build with docker on
non-Nvidia platforms.

BUG=None
TEST=./build_crosvm_base.sh
./build_crosvm.sh
./wrapped_smoke_test.sh

Change-Id: Ib2cc70d5bd98bcad25410ba54c0492491a3ea799
Signed-off-by: Jianxun Zhang <jianxun.zhang@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1842519
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Commit-Queue: Daniel Verkamp <dverkamp@chromium.org>
diff --git a/docker/Dockerfile.crosvm b/docker/Dockerfile.crosvm
index 77ede70..60ba5a9 100644
--- a/docker/Dockerfile.crosvm
+++ b/docker/Dockerfile.crosvm
@@ -15,7 +15,7 @@
 
 # Install nvidia proprietary drivers if requested.
 ARG NVIDIA_VERSION=
-RUN if [ -n NVIDIA_VERSION ]; then \
+RUN if [ -n "${NVIDIA_VERSION}" ]; then \
         nvidia_url=http://us.download.nvidia.com/XFree86/Linux-x86_64 && \
         nvidia_binary=NVIDIA-Linux-x86_64-"${NVIDIA_VERSION}".run && \
         curl -LO "${nvidia_url}"/"${NVIDIA_VERSION}"/"${nvidia_binary}" && \