Merge branch 'add/7.3' into 'master'

PHP 7.3 image

See merge request !4

php/7.3/Dockerfile

+FROM php:7.3-stretch
+MAINTAINER ethitter
+
+RUN apt-get update && apt-get -yqqf install lsb-release zip unzip subversion mysql-client libicu-dev libpng-dev libbz2-dev default-libmysqlclient-dev libzip-dev --fix-missing
+RUN docker-php-ext-install mbstring mysqli intl gd zip bz2
+
+RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer
+RUN chmod +x /usr/local/bin/composer
+
+ENV PATH=/root/.composer/vendor/bin:$HOME/.composer/vendor/bin:$PATH