fix embedded docker run instructions, add selinux help

closes #17

containers.go

@@ -58,7 +58,7 @@ func (x *ContainersCommand) Execute(args []string) error {
 		clientContainers, err := client.ListContainers(docker.ListContainersOptions{All: true})
 		if err != nil {
 			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'")
+				return fmt.Errorf("Unable to access Docker socket, please run like this:\n  docker run -it --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)
 			}

help.go

@@ -24,6 +24,18 @@ Dockviz supports connecting to the Docker daemon directly.  It defaults to
 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
 
+Running inside a Docker image:
+
+Dockviz can be run as a Docker image, like this:
+
+$ docker run -it --rm -v /var/run/docker.sock:/var/run/docker.sock nate/dockviz
+
+If you get an error that Dockviz is unable to access the Docker socket, even
+when specified, SELinux may be getting in the way.  See this issue comment for
+more information:
+
+https://github.com/justone/dockviz/issues/12#issuecomment-186682489
+
 Visualizing:
 
 Dockviz can visualize both images and containers. For more information on the

images.go

@@ -89,7 +89,7 @@ func (x *ImagesCommand) Execute(args []string) error {
 		ver, err := getAPIVersion(client)
 		if err != nil {
 			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'")
+				return fmt.Errorf("Unable to access Docker socket, please run like this:\n  docker run -it --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)
 			}