Update Dockerfile-71

fixing unexpected message "/bin/sh: 1: [: =: unexpected operator", #798.

php-fpm/Dockerfile-71

@@ -207,7 +207,7 @@ RUN if [ ${INSTALL_INTL} = true ]; then \
 #####################################
 
 ARG INSTALL_GHOSTSCRIPT=false
-RUN if [ ${GHOSTSCRIPT} = true ]; then \
+RUN if [ ${INSTALL_GHOSTSCRIPT} = true ]; then \
     # Install the ghostscript extension
     # for PDF editing
     apt-get -y update \