fill in help, add hint when running in docker

Dockerfile

@@ -1,3 +1,4 @@
 FROM scratch
 COPY dockviz /
+ENV IN_DOCKER true
 ENTRYPOINT ["/dockviz"]

containers.go

@@ -57,7 +57,11 @@ func (x *ContainersCommand) Execute(args []string) error {
 
 		clientContainers, err := client.ListContainers(docker.ListContainersOptions{All: true})
 		if err != nil {
-			return fmt.Errorf("Unable to connect: %s\nFor help, run 'dockviz help'", err)
+			if in_docker := os.Getenv("IN_DOCKER"); len(in_docker) > 0 {
+				return fmt.Errorf("Unable to access Docker socket, please run like this:\n  docker run --rm -v /var/run/docker.sock:/var/run/docker.sock nate/dockviz containers <args>\nFor more help, run 'dockviz help'")
+			} else {
+				return fmt.Errorf("Unable to connect: %s\nFor help, run 'dockviz help'", err)
+			}
 		}
 
 		var conts []Container

help.go

@@ -9,14 +9,34 @@ type HelpCommand struct {
 var helpCommand HelpCommand
 
 func (x *HelpCommand) Execute(args []string) error {
-	fmt.Println("stub help is here")
+	fmt.Println(`Dockviz: Visualizing Docker Data
+
+Connecting to Docker:
+
+Dockviz supports connecting to the Docker daemon directly.  It defaults to
+'unix:///var/run/docker.sock', but respects the following as well:
+
+* The 'DOCKER_HOST', 'DOCKER_CERT_PATH', and 'DOCKER_TLS_VERIFY' environment
+  variables, as set up by boot2docker or docker-machine.
+* Command line arguments (e.g. '--tlscacert'), like those that Docker itself
+  supports.
+
+Dockviz also supports receiving Docker image or container json data on standard
+input: curl -s http://localhost:4243/images/json?all=1 | dockviz images --tree
+
+Visualizing:
+
+Dockviz can visualize both images and containers. For more information on the
+options each subcommand supports, run them with the '--help' flag (e.g.
+'dockviz images --help').
+`)
 
 	return nil
 }
 
 func init() {
 	parser.AddCommand("help",
-		"Help for this tool.",
+		"Help for dockviz.",
 		"",
 		&helpCommand)
 }

images.go

@@ -59,7 +59,11 @@ func (x *ImagesCommand) Execute(args []string) error {
 
 		clientImages, err := client.ListImages(docker.ListImagesOptions{All: true})
 		if err != nil {
-			return fmt.Errorf("Unable to connect: %s\nFor help, run 'dockviz help'", err)
+			if in_docker := os.Getenv("IN_DOCKER"); len(in_docker) > 0 {
+				return fmt.Errorf("Unable to access Docker socket, please run like this:\n  docker run --rm -v /var/run/docker.sock:/var/run/docker.sock nate/dockviz images <args>\nFor more help, run 'dockviz help'")
+			} else {
+				return fmt.Errorf("Unable to connect: %s\nFor help, run 'dockviz help'", err)
+			}
 		}
 
 		var ims []Image