Skip geckodriver download for product builds

Dockerfile.product

@@ -22,8 +22,9 @@ RUN tar fx yarn-offline.tar
 # bootstrap yarn so we can install and run the other tools.
 RUN container-entrypoint npm install ./yarn-1.9.4.tgz
 
-# prevent download of chromedriver, sass binary, and node headers as part of module installs
+# prevent download of chromedriver, geckodriver, sass binary, and node headers as part of module installs
 ENV CHROMEDRIVER_SKIP_DOWNLOAD=true \
+    GECKODRIVER_SKIP_DOWNLOAD=true \
     SKIP_SASS_BINARY_DOWNLOAD_FOR_CI=true \
     NPM_CONFIG_TARBALL=$HOME/node-v8.9.4-headers.tar.gz