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 action, that is now obsolete.
Fixes#1035
