builder: qualify its own image with the registry host

Bare mesh-builder@sha256:... is only resolvable for a module with a build
section — the mesh's own build step rewrites the reference to a real
registry path as part of resolving build.artifacts. builder is handed
over, not built, so nothing ever rewrites it: pushed as written, docker
read it literally and tried Docker Hub. Took the live node's mesh-builder
down for the length of one push-and-fix (docker: pull access denied for
mesh-builder, repository does not exist). novox.internal:5100, not the
literal external IP docker inspect showed live, for the same reason
addresses generally don't get hardcoded in this catalogue.

modules/builder/module.json

@@ -50,7 +50,7 @@
       "id": "server",
       "type": "container",
       "name": "mesh-builder",
-      "image": "mesh-builder@sha256:42f5203a6838776447790d9e7d27f22a56e9462bdd25401645f22d188da56704",
+      "image": "novox.internal:5100/mesh-builder@sha256:42f5203a6838776447790d9e7d27f22a56e9462bdd25401645f22d188da56704",
       "env-file": [
         "/var/lib/mesh/builder/builder.env"
       ],