Update to latest mycroft-core version / automatic builds

[sanzoghenzo]

Is your feature request related to a problem? Please describe.

Lates mycroft core relase is 21.2.2.
The current version used by the docker container is 21.2.1, that has a bug in the mycroft-settings set command: is still tries to write to ~/.mycroft/mycroft.config where the other commands work on the ~/.config/mycroft/mycroft.conf file.

(this also means that the documentation needs to be updated to use the new configuration folders for docker volume binding)

Describe the solution you'd like

An update docker image published on docker hub.

An automated build workflow (via github actions or similar) for new releases would also be wonderful!

Describe alternatives you've considered

Obviusly I can build it myself, but I think the community would benefit for this to be done by the official channel