Add php extension php7-xmlreader only for php 7.1 and 7.2

Dockerfile

@@ -79,7 +79,7 @@ RUN echo 'hosts: files dns' > /etc/nsswitch.conf && \
     if [ "${PHP_VERSION}" = "7.0" ]; then \
       echo "no extra packages required" ; \
     else \
-      apk add --no-cache php7-fileinfo php7-tokenizer php7-simplexml php7-xmlwriter ; \
+      apk add --no-cache php7-fileinfo php7-tokenizer php7-simplexml php7-xmlreader php7-xmlwriter ; \
     fi && \
     rm /etc/apk/repositories && \
     mv /etc/apk/repositories.org /etc/apk/repositories && \