Add the extdom and libxml2 to the PHP container.

dev/php/Dockerfile

@@ -6,9 +6,17 @@ RUN apt-get update && \
     apt-get install -y libonig-dev \
                        zip \
                        libzip-dev \
-                       libpng-dev
+                       libpng-dev \
+                       libxml2-dev
 
-RUN docker-php-ext-install pdo pdo_mysql mbstring gd zip
+RUN docker-php-ext-install pdo \
-RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
+                           pdo_mysql \
+                           mbstring \
+                           gd \
+                           zip \
+                           dom
+
+RUN curl -sS https://getcomposer.org/installer | \
+    php -- --install-dir=/usr/local/bin --filename=composer