docker: make log rotation a preference, 100m x 10 by default, judged by a step before daemon.json takes it (novox/hq issue 452)

The controller pastes a setting into daemon.json as text and the runtime is its
only reader, so a bad size would surface as a runtime that does not start and a
quote could add any key. A step refuses the value first and gates the module.
The defaults are the one hand-set machine's, so it does not change; nothing
restarts the runtime, which takes the keys at its next start.

modules/docker/cmd/docker-tools/main.go

@@ -16,6 +16,20 @@ import (
 )
 
 func main() {
+	// Started as `docker-tools check-log-rotation` it is the module's step that judges the log rotation before
+	// daemon.json takes it (logrotation.go, novox/hq issue 452): run as root by the host, reading the file
+	// DOCKER_LOG_ROTATION names. With no argument, the runtime's tools bundle.
+	if len(os.Args) > 1 {
+		if os.Args[1] != "check-log-rotation" || len(os.Args) != 2 {
+			fmt.Fprintln(os.Stderr, "usage: docker-tools [check-log-rotation]")
+			os.Exit(2)
+		}
+		if err := checkLogRotationFile(os.Getenv("DOCKER_LOG_ROTATION")); err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		return
+	}
 	// An empty name serves as the module the runtime names (MESH_SERVED_MODULE): docker.
 	if err := stdio.Serve("", tools(NewClient())); err != nil {
 		fmt.Fprintln(os.Stderr, err)