Use start-sdk's docker-images/<arch>.tar convention

start-sdk pack discovers per-arch tarballs at docker-images/
x86_64.tar and docker-images/aarch64.tar by filename convention,
not via a manifest assets entry. Dropping the invalid
'docker-images: image.tar' assets key and reworking the Makefile
to build one type=docker tarball per arch with arm/x86 single-
arch convenience targets.

manifest.yaml

@@ -31,7 +31,6 @@ assets:
   license: LICENSE
   icon: icon.png
   instructions: instructions.md
-  docker-images: image.tar
 
 main:
   type: docker