dont run conditional, just use the syntax for default vars in dockerfile

Author: ddavison

NodeChrome/Dockerfile

@@ -13,13 +13,12 @@ USER root
 #       latest (equivalent to google-chrome-stable)
 #       google-chrome-beta  (pull latest beta)
 #============================================
-ENV CHROME_VERSION latest
-RUN if $CHROME_VERSION -eq "latest"; then export CHROME="google-chrome-stable"; else export CHROME="${CHROME_VERSION}"; fi
+ENV CHROME_VERSION "google-chrome-stable"
 RUN wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add - \
   && echo "deb http://dl.google.com/linux/chrome/deb/ stable main" >> /etc/apt/sources.list.d/google-chrome.list \
   && apt-get update -qqy \
   && apt-get -qqy install \
-    $CHROME \
+    ${CHROME_VERSION:-google-chrome-stable} \
   && rm /etc/apt/sources.list.d/google-chrome.list \
   && rm -rf /var/lib/apt/lists/*