New docker image versioning, new repository ghcr.io

[lukaszbudnik]

As of v2021.0.3 version, there are 3 important changes made to docker image management.

1. migrator images are now available in ghcr.io: https://github.com/lukaszbudnik/migrator/pkgs/container/migrator in addition to default Docker Hub repository: https://hub.docker.com/r/lukasz/migrator 🎉 🥳
2. the "v" prefix is dropped, apart from the default {major}.{minor}.{patch} tag, the following additional tags are automatically created {major}.{minor} and {major}.
3. automatic merges to the main branch no longer trigger building and releasing latest tag. Instead, there is a new edge tag pushed.

As an example, publishing the v2021.0.3 release on GitHub resulted in creating the following 4 docker images: 2021.0.3, 2021.0, 2021, and latest. Version 2021.0 will contain all versions matching 2021.0.X, version 2021 will contain all versions matching 2021.X.X. latest will always point to the latest version.