support specifying docker host endpoint

cli.go

@@ -11,6 +11,7 @@ type GlobalOptions struct {
 	TLSCert   string `long:"tlscert" value-name:"~/.docker/cert.pem" description:"Path to TLS certificate file"`
 	TLSKey    string `long:"tlskey" value-name:"~/.docker/key.pem" description:"Path to TLS key file"`
 	TLSVerify bool   `long:"tlsverify" description:"Use TLS and verify the remote"`
+	Host      string `long:"host" short:"H" value-name:"unix:///var/run/docker.sock" description:"Docker host to connect to"`
 }
 
 var globalOptions GlobalOptions

containers.go

@@ -51,6 +51,9 @@ func (x *ContainersCommand) Execute(args []string) error {
 	} else {
 
 		client, err := connect()
+		if err != nil {
+			return err
+		}
 
 		clientContainers, err := client.ListContainers(docker.ListContainersOptions{All: true})
 		if err != nil {

util.go

@@ -11,8 +11,13 @@ import (
 func connect() (*docker.Client, error) {
 
 	// grab directly from docker daemon
-	endpoint := os.Getenv("DOCKER_HOST")
-	if len(endpoint) == 0 {
+	var endpoint string
+	if env_endpoint := os.Getenv("DOCKER_HOST"); len(env_endpoint) > 0 {
+		endpoint = env_endpoint
+	} else if len(globalOptions.Host) > 0 {
+		endpoint = globalOptions.Host
+	} else {
+		// assume local socket
 		endpoint = "unix:///var/run/docker.sock"
 	}