Add geckodriver

Author: jacebrowning

Dockerfile

@@ -11,3 +11,9 @@ RUN sudo wget -q -O /usr/bin/lein \
 https://raw.githubusercontent.com/technomancy/leiningen/stable/bin/lein && \
 sudo chmod +x /usr/bin/lein && \
 lein --version
+
+# Firefox WebDriver
+
+RUN curl -L https://github.com/mozilla/geckodriver/releases/download/v0.17.0/geckodriver-v0.17.0-linux64.tar.gz \
+| sudo tar xz -C /usr/local/bin && \
+geckodriver --version