Run builds and checks in a per-node build slice with CPU, IO and memory limits (issue 1798)

cmd/mesh-builder/main.go

@@ -64,6 +64,8 @@ is dialled except the broker.
   MESH_NPM_TOKEN     the token for it, likewise
   MESH_NPM_SCOPE     the scope it answers for (default: @novox)
   MESH_WORKSPACE     where to clone and build (default: a temporary directory)
+  MESH_BUILD_CGROUP_PARENT  the slice every container a build or check starts joins, which carries the
+                     machine's limits on builds (novox/hq issue 1798; default: the runtime's own)
 
 It also builds one module and stops, which is how a mesh is raised — before there is a
 broker to take work from or a registry to publish into:
@@ -137,6 +139,10 @@ func run() error {
 	if err != nil {
 		return err
 	}
+	// A build and a check run below the machine's own work (novox/hq issue 1798): the agent itself at the
+	// lowest priority, which its git and go children inherit, and every container in the build slice.
+	_ = syscall.Setpriority(syscall.PRIO_PROCESS, 0, 19)
+	builder.CgroupParent = strings.TrimSpace(os.Getenv("MESH_BUILD_CGROUP_PARENT"))
 	workspace := os.Getenv("MESH_WORKSPACE")
 	if workspace == "" {
 		workspace = os.TempDir() + "/mesh-builder"