added a few extenstions for PHP (#7)

considering that Grav can be used internationally, i've added the `intl` extension and it's auto configuration. 
also i've enabled the `intl` extenstion

- at least that is what i believe i did considering https://github.com/docker-library/php/issues/57#issuecomment-238077158

Co-authored-by: Andy Miller <1084697+rhukster@users.noreply.github.com>

Dockerfile

@@ -13,9 +13,15 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libyaml-dev \
     libzip4 \
     libzip-dev \
+    zlib1g-dev \
+	  libicu-dev \
+	  g++ \
+	  git \
 	  cron \
 	  vim \
     && docker-php-ext-install opcache \
+    && docker-php-ext-configure intl \
+    && docker-php-ext-install intl \
     && docker-php-ext-configure gd --with-freetype-dir=/usr/include/ --with-jpeg-dir=/usr/include/ \
     && docker-php-ext-install -j$(nproc) gd \
     && docker-php-ext-install zip
@@ -32,6 +38,7 @@ RUN { \
 		echo 'opcache.enable_cli=1'; \
 		echo 'upload_max_filesize=128M'; \
 		echo 'post_max_size=128M'; \
+		echo 'extension=intl.so'; \
 	} > /usr/local/etc/php/conf.d/php-recommended.ini
 
 RUN pecl install apcu \