updated xdebug

- see also https://bugs.xdebug.org/bug_view_page.php?bug_id=00001642

Author: schmunk42

php/Dockerfile-fpm

@@ -72,7 +72,7 @@ RUN printf "\n" | pecl install \
 RUN cd /tmp && \
     git clone git://github.com/xdebug/xdebug.git && \
     cd xdebug && \
-    git checkout tags/2.7.0beta1 && \
+    git checkout tags/2.8.0 && \
     phpize && \
     ./configure --enable-xdebug && \
     make && \