fix GD on PHP 5.6 as well

Mahmoudz · Mahmoudz · commit dd288bf61d7f · 2016-08-16T13:44:49.000+03:00
diff --git a/Dockerfile-56 b/Dockerfile-56
@@ -48,4 +48,5 @@ RUN docker-php-ext-install gd && \
     docker-php-ext-configure gd \
         --enable-gd-native-ttf \
         --with-jpeg-dir=/usr/lib \
-        --with-freetype-dir=/usr/include/freetype2
+        --with-freetype-dir=/usr/include/freetype2 && \
+    docker-php-ext-install gd
