Feature/dockerize

[turicas]

This PR adds a Dockerfile, Docker-related make targets and docs. It still does not tag the image based on git tags but the maintainer can do it manually in an easy way.

Note 1: it uses the Python Alpine-based image so we save space when generating the image (faster to download/run the image from Docker Hub).
Note 2: you may replace turicas/pyp5js with your Docker Hub username/image from the example on docs/index.md.

[turicas]

I've rebased this PR to the new develop.