allowing php-worker to install gmp extension (#2152)

php-worker/Dockerfile

@@ -103,6 +103,14 @@ RUN if [ $INSTALL_GHOSTSCRIPT = true ]; then \
     apk --update add ghostscript \
 ;fi
 
+#Install GMP package:
+ARG INSTALL_GMP=false
+RUN if [ ${INSTALL_GMP} = true ]; then \
+   apk add --update --no-cache gmp gmp-dev \
+   && docker-php-ext-install gmp \
+;fi
+
+
 RUN rm /var/cache/apk/* \
     && mkdir -p /var/www