Docker swarm: enable unix socket

Fixes #7603

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

discovery/dockerswarm/dockerswarm.go

@@ -88,16 +88,12 @@ type Discovery struct {
 
 // NewDiscovery returns a new Discovery which periodically refreshes its targets.
 func NewDiscovery(conf *SDConfig, logger log.Logger) (*Discovery, error) {
+	var err error
 	d := &Discovery{
 		port: conf.Port,
 		role: conf.Role,
 	}
 
-	rt, err := config_util.NewRoundTripperFromConfig(conf.HTTPClientConfig, "dockerswarm_sd", false)
-	if err != nil {
-		return nil, err
-	}
-
 	// This is used in tests. In normal situations, it is set when Unmarshaling.
 	if conf.url == nil {
 		conf.url, err = url.Parse(conf.Host)
@@ -106,15 +102,29 @@ func NewDiscovery(conf *SDConfig, logger log.Logger) (*Discovery, error) {
 		}
 	}
 
-	d.client, err = client.NewClientWithOpts(
+	opts := []client.Opt{
 		client.WithHost(conf.Host),
-		client.WithHTTPClient(&http.Client{
-			Transport: rt,
-			Timeout:   time.Duration(conf.RefreshInterval),
-		}),
-		client.WithScheme(conf.url.Scheme),
 		client.WithAPIVersionNegotiation(),
-	)
+	}
+
+	// There are other protocols that HTTP supported by the Docker daemon, like
+	// unix, which do not require an http client. Passing an http client to the
+	// docker client makes those non-http requests fail.
+	if conf.url.Scheme == "http" || conf.url.Scheme == "https" {
+		rt, err := config_util.NewRoundTripperFromConfig(conf.HTTPClientConfig, "dockerswarm_sd", false)
+		if err != nil {
+			return nil, err
+		}
+		opts = append(opts,
+			client.WithHTTPClient(&http.Client{
+				Transport: rt,
+				Timeout:   time.Duration(conf.RefreshInterval),
+			}),
+			client.WithScheme(conf.url.Scheme),
+		)
+	}
+
+	d.client, err = client.NewClientWithOpts(opts...)
 	if err != nil {
 		return nil, fmt.Errorf("error setting up docker swarm client: %w", err)
 	}