A build says which machine made it, in the mesh's name for that machine

An event whose origin reads a container id names something no other module
can look up. The mesh already knows the answer, and a module's environment
file is a file resource, so ${machine:name} reaches it with no composer change.

cmd/mesh-builder/main.go

@@ -82,9 +82,18 @@ func run() error {
 	if workspace == "" {
 		workspace = os.TempDir() + "/mesh-builder"
 	}
-	on, err := os.Hostname()
+	// **The mesh's name for this machine, not the container's.** A build is reported to the rest
-	if err != nil {
+	// of the mesh, and a report whose origin reads `104cb10e105b` names something no other module
-		on = "a build machine"
+	// can look up. The mesh already knows the answer and has a way to say it — `${machine:name}`
+	// in the environment file this module is handed — so the hostname is only what is left when
+	// nobody said.
+	on := os.Getenv("MESH_NODE")
+	if on == "" {
+		hostname, err := os.Hostname()
+		if err != nil {
+			return fmt.Errorf("this build machine has no name: nothing said MESH_NODE and the host would not say either: %w", err)
+		}
+		on = hostname
 	}
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)