Adds php-gd and mysqli to dockerfile

Dockerfile

 FROM php:5.6-apache
 
-RUN apt-get update && apt-get install -y php5-gd php5-mysql
+RUN apt-get update && apt-get install -y libpng12-dev libjpeg-dev \
+	&& rm -rf /var/lib/apt/lists/* \
+	&& docker-php-ext-configure gd --with-png-dir=/usr --with-jpeg-dir=/usr \
+	&& docker-php-ext-install gd
+
+RUN docker-php-ext-install mysqli
 
 COPY htdocs/ /var/www/html/