Change FIREFOX_VERSION from ENV to ARG

Author: Francois-Xavier Gentilhomme

NodeFirefox/Dockerfile.txt

@@ -5,7 +5,7 @@ USER root
 #=========
 # Firefox
 #=========
-ENV FIREFOX_VERSION 47.0.1
+ARG FIREFOX_VERSION=47.0.1
 RUN apt-get update -qqy \
   && apt-get -qqy --no-install-recommends install firefox \
   && rm -rf /var/lib/apt/lists/* \
@@ -20,7 +20,7 @@ RUN apt-get update -qqy \
 #============
 # GeckoDriver
 #============
-ENV GECKODRIVER_VERSION 0.10.0
+ARG GECKODRIVER_VERSION=0.10.0
 RUN wget --no-verbose -O /tmp/geckodriver.tar.gz https://github.com/mozilla/geckodriver/releases/download/v$GECKODRIVER_VERSION/geckodriver-v$GECKODRIVER_VERSION-linux64.tar.gz \
   && rm -rf /opt/geckodriver \
   && tar -C /opt -zxf /tmp/geckodriver.tar.gz \