Misc de-duplication and cleanups
This splits out the de-dup/cleanups out of !81 (merged) and adds a few more.
- Drop duplicate parallel/matrix
- De-duplicate podman tag/push commands
- Move shell scripts out of the Makefile
- scripts/make-rootfs.sh: drop pacman gnupg removal
- scripts/make-rootfs.sh: move non-wrapper calls further up
- scripts/make-dockerfile.sh: tweak sha256 sum handling
- scripts/make-dockerfile.sh: rework TEMPLATE_ROOTFS_DOWNLOAD handling
- scripts/make-dockerfile.sh: drop TEMPLATE_ROOTFS_RELEASE_URL
- Use pattern rules in the makefile
- scripts/make-dockerfile.sh: reuse in the release stage
- Parametrise Github/Dockerhub dockerfile
The above might be overly verbose split to be on the safe side. Squashing it trivial ;-)