Add Dockerfile for quick and easy testing

Not that it's hard to set up an environment to test
this library in, but perhaps you just can't be bothered
to mess up your system installing software and versions
you don't otherwise need.

Build the image:
$ docker build -t minify .

Run the tests:
$ docker run --rm --name minify minify

Or if you plan on testing/changing the code and don't
want to rebuild the image, just load it as a volume:
$ docker run --rm --name minify -v ~/Sites/minify:/var/www minify

Dockerfile

@@ -0,0 +1,24 @@
+FROM php:cli
+
+# install composer and a bunch of dependencies
+RUN apt-get update && apt-get install -y git curl zip unzip zlib1g-dev
+RUN docker-php-ext-install zip
+RUN docker-php-ext-install pcntl
+RUN curl -sS https://getcomposer.org/installer | php
+RUN mv composer.phar /usr/local/bin/composer
+
+# pull in code
+WORKDIR /var/www
+COPY . .
+
+# install dependencies
+RUN composer install
+
+# to support loading the directory as volume, we'll move vendor out of the way so it
+# doesn't get overwritten by more recent code; we'll put it back before running anything
+RUN mv vendor ../vendor
+RUN echo 'cp -r /var/vendor /var/www/vendor && exec "$@"' > /etc/run.sh
+RUN chmod +x /etc/run.sh
+ENTRYPOINT ["/bin/sh", "/etc/run.sh"]
+
+CMD ["vendor/bin/phpunit"]