add GitHub action for TEI releases

[peterstadler]

when a TEI release is created the TEIGarage Docker image needs to pick up the new Stylesheets and Source.

@anneferger and I discussed the following workflow:

1. a TEI release triggers a new git tag with an incremented minor version number (this could be done via a GitHub action in the TEI repo; otherwise manually)
2. the commit message of the tagging must start with "TEI P5 Release"
3. a Github action in the TEIGarage repo listens on refs/tags and the above prefix, finally building the Docker image with the "latest" and the version tag

[peterstadler]

maybe have a look at https://github.com/softprops/action-gh-release for creating GitHub Releases via GitHub actions.