Add the ability to spin up Hetzner servers from custom snapshots

This comes in handy when using tools such as Packer to generate customized images.

cluster-autoscaler/cloudprovider/hetzner/README.md

@@ -5,11 +5,17 @@ The cluster autoscaler for Hetzner Cloud scales worker nodes.
 # Configuration
 
 `HCLOUD_TOKEN` Required Hetzner Cloud token.
+
 `HCLOUD_CLOUD_INIT` Base64 encoded Cloud Init yaml with commands to join the cluster, Sample [examples/cloud-init.txt for (Kubernetes 1.20.1)](examples/cloud-init.txt)
-`HCLOUD_IMAGE` Defaults to `ubuntu-20.04`, @see https://docs.hetzner.cloud/#images
+
+`HCLOUD_IMAGE` Defaults to `ubuntu-20.04`, @see https://docs.hetzner.cloud/#images. Alternatively, supply the description of a custom snapshot and the most recent (snapshot) image will be used. Useful when building images with tools such as Packer.
+
 `HCLOUD_NETWORK` Default empty , The name of the network that is used in the cluster , @see https://docs.hetzner.cloud/#networks
+
 `HCLOUD_SSH_KEY` Default empty , This SSH Key will have access to the fresh created server, @see https://docs.hetzner.cloud/#ssh-keys
+
 Node groups must be defined with the `--nodes=<min-servers>:<max-servers>:<instance-type>:<region>:<name>` flag.
+
 Multiple flags will create multiple node pools. For example:
 ```
 --nodes=1:10:CPX51:FSN1:pool1

cluster-autoscaler/cloudprovider/hetzner/hcloud-go/hcloud/image.go

@@ -117,13 +117,41 @@ func (c *ImageClient) GetByName(ctx context.Context, name string) (*Image, *Resp
 	return images[0], response, err
 }
 
+// GetBySnapshot retrieves the most recent image given a snapshot description. If the image cannot be found, nil is returned.
+func (c *ImageClient) GetBySnapshot(ctx context.Context, name string) (*Image, *Response, error) {
+	if name == "" {
+		return nil, nil, nil
+	}
+
+	images, response, err := c.List(ctx, ImageListOpts{
+		Type:   []ImageType{ImageTypeSnapshot},
+		Status: []ImageStatus{ImageStatusAvailable},
+		Sort:   []string{"created:desc"},
+	})
+
+	for _, i := range images {
+		if i.Description == name {
+			return i, response, err
+		}
+	}
+
+	return nil, response, err
+}
+
 // Get retrieves an image by its ID if the input can be parsed as an integer, otherwise it
 // retrieves an image by its name. If the image does not exist, nil is returned.
 func (c *ImageClient) Get(ctx context.Context, idOrName string) (*Image, *Response, error) {
 	if id, err := strconv.Atoi(idOrName); err == nil {
-		return c.GetByID(ctx, int(id))
+		if image, resp, err := c.GetByID(ctx, int(id)); err == nil && image != nil {
+			return image, resp, nil
+		}
 	}
-	return c.GetByName(ctx, idOrName)
+
+	if image, resp, err := c.GetByName(ctx, idOrName); err == nil && image != nil {
+		return image, resp, nil
+	}
+
+	return c.GetBySnapshot(ctx, idOrName)
 }
 
 // ImageListOpts specifies options for listing images.

cluster-autoscaler/cloudprovider/hetzner/hetzner_node_group.go

@@ -348,12 +348,18 @@ func serverTypeAvailable(manager *hetznerManager, instanceType string, region st
 
 func createServer(n *hetznerNodeGroup) error {
 	StartAfterCreate := true
+
+	image, _, err := n.manager.client.Image.Get(n.manager.apiCallContext, n.manager.image)
+	if err != nil {
+		return fmt.Errorf("unable to find image named %s: %v", n.manager.image, err)
+	}
+
 	opts := hcloud.ServerCreateOpts{
 		Name:             newNodeName(n),
 		UserData:         n.manager.cloudInit,
 		Location:         &hcloud.Location{Name: n.region},
 		ServerType:       &hcloud.ServerType{Name: n.instanceType},
-		Image:            &hcloud.Image{Name: n.manager.image},
+		Image:            image,
 		StartAfterCreate: &StartAfterCreate,
 		Labels: map[string]string{
 			nodeGroupLabel: n.id,