Add files to run php74 tests inside a docker container

tests/dockerfiles/php74/Dockerfile

@@ -0,0 +1,19 @@
+FROM php:7.4-cli
+ENV DEBIAN_FRONTEND noninteractive
+
+RUN apt-get --assume-yes --quiet update
+
+RUN apt-get --assume-yes --quiet install git && \
+    apt-get --assume-yes --quiet install libpng-dev && \
+    apt-get --assume-yes --quiet install rsync && \
+    apt-get --assume-yes --quiet install zip
+
+RUN curl --silent --show-error "https://getcomposer.org/installer" | php && \
+    mv "composer.phar" "/usr/local/bin/composer" && \
+    composer global require --no-interaction "phpunit/phpunit"
+
+RUN docker-php-ext-configure gd && \
+    docker-php-ext-install gd
+
+ENV PATH /root/.composer/vendor/bin:$PATH
+CMD ["bash"]