Tests - Docker - add required libraries, PHP extensions & optimize size of Docker's image

.docker/config/Dockerfile

@@ -3,17 +3,27 @@ FROM php:5.6-cli
 #
 # Tools & libraries
 #
-RUN apt-get update && apt-get install -y \
-    libicu-dev
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        git \
+        zip \
+        unzip \
+        zlib1g-dev \
+        libicu-dev \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 #
 # PHP Extensions
 #
 RUN docker-php-ext-install \
-    intl
+    zip \
+    intl \
+    mbstring
 
 #
-# Timezone
+# PHP configuration:
+# - timezone
 #
 ARG TIMEZONE
 RUN echo "date.timezone = $TIMEZONE" >> /usr/local/etc/php/php.ini
@@ -32,11 +42,13 @@ RUN pear channel-discover pear.phing.info \
     && pear install [--alldeps] phing/phing
 
 #
-# Composer
+# Composer + https://packagist.org/packages/hirak/prestissimo package
 #
 RUN php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" \
     && php -r "if (hash_file('SHA384', 'composer-setup.php') === \
     '544e09ee996cdf60ece3804abc52599c22b1f40f4323403c44d44fdfdd586475ca9813a858088ffbc1f233e9b180f061') { echo \
     'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;" \
     && php composer-setup.php --install-dir=/usr/local/bin --filename=composer \
-    && php -r "unlink('composer-setup.php');"
+    && php -r "unlink('composer-setup.php');" \
+    && composer global require --no-plugins --no-scripts hirak/prestissimo \
+    && rm -rf /root/.composer/cache/*