The `build-push-image.yml` reusable workflow can generate keypairs and
sign the container images with them.
This is only used by the CI, to test that a valid signature is actually
detected as such.
Create a reusable GitHub Actions workflow that does the following:
1. Create a multi-architecture container image for Dangerzone, instead
of having two different tarballs (or no option at all)
2. Build the Dangerzone container image on our supported architectures
(linux/amd64 and linux/arm64). It so happens that GitHub also offers
ARM machine runners, which speeds up the build.
3. Combine the images from these two architectures into one, multi-arch
image.
4. Generate provenance info for each manifest, and the root manifest
list.
5. Check the image's reproduciblity.
Also, remove an older CI job for checking the reproducibility of the
image, which is now obsolete.
Fixes#1035
