v1.5.6: Fix docker, update license and dependencies

This project now uses the GPLv3 license. This is a marginally more restrictive license than MIT - if you make changes to the source code or incorporate it into a larger project you should also publish that source code.
This release also updates dependencies - including resolving CVEs - and includes some chores in the readme.

What's Changed

• Update license by @Neztore in #54

Full Changelog: v1.5.5...v1.5.6