Dockerfile: Use correct libpng package for Debian Stretch

Dockerfile

@@ -3,7 +3,7 @@ FROM php:apache
 RUN apt-get update && apt-get install -y \
         libfreetype6-dev \
         libjpeg62-turbo-dev \
-        libpng12-dev \
+        libpng-dev \
         wget \
         zip \
         unzip && \