build.on takes {arg, image@sha256:…} beside {arg, module, artifact}: the image is
copied into the mesh's registry before the build (ADR 0096) and the recipe reads the
copy from the argument. A FROM or COPY --from naming a registry image the manifest
did not declare is refused before the build, naming it and the remedy; stages,
declared arguments and scratch are not fetches (novox/hq 04-ISSUES/064, ADR 0097).