I'd like to upgrade blubber version in our docker images to the latest version.
The latest version - v0.23.0 - now uses a virtualenv for all python operations so it doesnt affect system python packages. (more details in release Changelog).

Support for the`use-system-flag` option has been removed which means that our images will fail to build.
If needed we can use the option use-system-site-packages option which will run python -m venv --system-site-packages when creating the virtualenv.
It also seems that we need to install python3-venv wherever we use a standard debian distro.