Fix php5 Unable to locate gmp.h

php-fpm/Dockerfile

@@ -209,6 +209,9 @@ ARG INSTALL_GMP=false
 RUN if [ ${INSTALL_GMP} = true ]; then \
     # Install the GMP extension
 	  apt-get install -y libgmp-dev && \
+    if [ $(php -r "echo PHP_MAJOR_VERSION;") = "5" ]; then \
+      ln -s /usr/include/x86_64-linux-gnu/gmp.h /usr/include/gmp.h \
+    ;fi && \
     docker-php-ext-install gmp \
 ;fi