Add php7-simplexml

Dockerfile

@@ -19,7 +19,8 @@ RUN apk update && \
     \
     apk add --no-cache bash python nodejs git unzip \
         php7 php7-phar php7-json php7-dom php7-gd php7-mbstring php7-openssl \
-        php7-pdo php7-curl php7-xml php7-zip php7-session php7-ctype php7-tokenizer && \
+        php7-pdo php7-curl php7-xml php7-zip php7-session php7-ctype \
+        php7-tokenizer php7-simplexml && \
     ln -s /usr/bin/php7 /usr/local/bin/php &&\
     \
     wget -q https://github.com/docker/compose/releases/download/${compose_version}/docker-compose-`uname -s`-`uname -m` -O /usr/local/bin/docker-compose && \