Find all references to the `container.tar.gz` file, and replace them
with references to `container.tar`. Moreover, remove the `--no-save`
argument of `build-image.py` since we now always save the image.
Finally, fix some stale references to Poetry, which are not necessary
anymore.
Add a new script called `sign-assets.py`, which produces the hash of all
the Dangerzone assets for a release (Windows/macOS installers, container
image), and signs them individually.
Also update our RELEASE.md document, to incorporate this script into our
release workflow.
