Replace libpng12-dev with libpng-dev

libpng12-dev has been dropped in 18.04

See:
- https://askubuntu.com/questions/991706/e-package-libpng12-dev-has-no-installation-candidate
- https://askubuntu.com/questions/840257/e-package-libpng12-0-has-no-installation-candidate-ubuntu-16-10-gnome/840268

Author: pschmitt

Dockerfile

@@ -4,7 +4,7 @@ ENV BOOKSTACK=BookStack \
     BOOKSTACK_VERSION=0.22.0 \
     BOOKSTACK_HOME="/var/www/bookstack"
 
-RUN apt-get update && apt-get install -y git zlib1g-dev libfreetype6-dev libjpeg62-turbo-dev libmcrypt-dev libpng12-dev wget libldap2-dev libtidy-dev \
+RUN apt-get update && apt-get install -y git zlib1g-dev libfreetype6-dev libjpeg62-turbo-dev libmcrypt-dev libpng-dev wget libldap2-dev libtidy-dev \
    && docker-php-ext-install pdo pdo_mysql mbstring zip tidy \
    && docker-php-ext-configure ldap --with-libdir=lib/x86_64-linux-gnu/ \
    && docker-php-ext-install ldap \