Added Dockerfile to run clx as a container.

[stefins]

Hi,

This PR add support for circumflex to run in Docker based environments

I have tested this on podman. Because we need latest version of less I have to build it from source for a small container image.

The resulting image is of size 34.6 MB.

Known issues:

The terminal colors doesn't seem to work when I run it inside the container.

Thanks :)

[bensadeh]

Hi @stefins !

Thanks for taking the time to put this PR together! Very useful to add the Dockerfile for people to try out. I will merge this as-is and update the readme with instructions on how to use once the colors are working.