The _version.py file is no longer kept in version control and instead is generated when running pip install -e . or by hatch build when packaging the distribution for PyPI/Conda Forge. So it's possible to clone the repo and never install it, so we catch the ImportError and use a null version so imports at least work.

This was stopping the .git directory from being copied in which is needed to correctly resolve the version number. To avoid this blowing up the container size I've switched the Dockerfile to do a multi-stage build where we build the distributions and then only copy those over to the final container image.