Fix libpng package reference

images/php/5.6/Dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update && apt-get install -y \
   libicu-dev \
   libjpeg62-turbo-dev \
   libmcrypt-dev \
-  libpng12-dev \
+  libpng-dev \
   libxslt1-dev \
   lsof \
   mysql-client \

images/php/7.0/Dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update && apt-get install -y \
   libicu-dev \
   libjpeg62-turbo-dev \
   libmcrypt-dev \
-  libpng12-dev \
+  libpng-dev \
   libxslt1-dev \
   lsof \
   mysql-client \

images/php/7.1/Dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update && apt-get install -y \
   libicu-dev \
   libjpeg62-turbo-dev \
   libmcrypt-dev \
-  libpng12-dev \
+  libpng-dev \
   libxslt1-dev \
   lsof \
   mysql-client \