fix build-args INSTALL_GHOSTSCRIPT were not consumed (laradock#2452)

Author: bestlong

php-worker/Dockerfile

@@ -142,6 +142,7 @@ RUN if [ $INSTALL_PHALCON = true ]; then \
     && rm -rf /tmp/cphalcon* \
 ;fi
 
+ARG INSTALL_GHOSTSCRIPT=false
 RUN if [ $INSTALL_GHOSTSCRIPT = true ]; then \
     apk --update add ghostscript \
 ;fi