create the image if it doesn't exist on host

Docker-DCO-1.1-Signed-off-by: Aaron Feng aaron.feng@gmail.com (github: aaronfeng)

backends/dockerclient.go

@@ -106,21 +106,25 @@ func (b *dockerClientBackend) ls() ([]string, error) {
 	return names, nil
 }
 
-func (b *dockerClientBackend) spawn(cmd ...string) (beam.Sender, error) {
+func (b *dockerClientBackend) createContainer(cmd ...string) (beam.Sender, error) {
   if len(cmd) != 1 {
 		return nil, fmt.Errorf("dockerclient: spawn takes exactly 1 argument, got %d", len(cmd))
 	}
+
 	resp, err := b.client.call("POST", "/containers/create", cmd[0])
 	if err != nil {
 		return nil, err
 	}
+
 	respBody, err := ioutil.ReadAll(resp.Body)
 	if err != nil {
 		return nil, err
 	}
-	if resp.StatusCode != 201 {
+
+	if resp.StatusCode == 404 {
 		return nil, fmt.Errorf("expected status code 201, got %d:\n%s", resp.StatusCode, respBody)
   }
+
 	var respJson struct{ Id string }
 	if err = json.Unmarshal(respBody, &respJson); err != nil {
 		return nil, err
@@ -128,6 +132,57 @@ func (b *dockerClientBackend) spawn(cmd ...string) (beam.Sender, error) {
 	return b.newContainer(respJson.Id), nil
 }
 
+func (b *dockerClientBackend) createImage(cmd ...string) error {
+	if len(cmd) != 1 {
+		return fmt.Errorf("dockerclient: image name is the only expected argument, got %d", len(cmd))
+	}
+
+	var respJson struct{ Image string }
+  if err := json.Unmarshal([]byte(cmd[0]), &respJson); err != nil {
+		return err
+	}
+
+  imageTag := strings.Split(respJson.Image, ":")
+
+  var tag = "latest"
+  image := imageTag[0]
+
+  if len(imageTag) > 1 {
+    tag = imageTag[1]
+  }
+
+  url := fmt.Sprintf("/images/create?fromImage=%s&tag=%s", image, tag)
+
+  resp, err := b.client.call("POST", url, "")
+	if err != nil {
+		return err
+	}
+
+  _, err = ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return err
+	}
+
+  return nil
+}
+
+func (b *dockerClientBackend) spawn(cmd ...string) (beam.Sender, error) {
+  sender, err := b.createContainer(cmd...)
+
+  if err != nil {
+    err = b.createImage(cmd...)
+    if err != nil {
+      return sender, err
+    }
+    sender, err = b.createContainer(cmd...)
+    if err != nil {
+      return sender, err
+    }
+  }
+
+	return sender, nil
+}
+
 func (b *dockerClientBackend) newContainer(id string) beam.Sender {
 	c := &container{backend: b, id: id}
 	instance := beam.NewServer()