Less is a CSS pre-processor, meaning that it extends the CSS language, adding features that allow variables, mixins, functions and many other techniques that allow you to make CSS that is more maintainable, themeable and extendable.
This image contains the
less CLI including the
clean-css plugin and allows to use it out of the box.
docker run --rm --interactive --tty \ --volume $PWD:/app \ finalgene/lessc ...
Take a look at the starting guide to get detailed information about less.