Feature/SK-164 Update gh workflows, imports and formatting checks

Status

Major change to gh workflow:

gh container registry used instead of docker-hub to build and push images
These images are updated on push and pull requests againts main and develop branches only
Added build of jupyter-stackn image, including mlflow, minio and stackn CLI
Code-checks introduced, including checking imports (isort), formatting (autopep8) and unit tests (unittest for CLI, django test framework for Studio)