clarify automatic service creation/assignment with labels

pkg/provider/docker/config_test.go

@@ -798,6 +798,57 @@ func Test_buildConfiguration(t *testing.T) {
 				},
 			},
 		},
+		{
+			desc: "one router, one specified but undefined service -> specified one is assigned, but automatic is created instead",
+			containers: []dockerData{
+				{
+					ServiceName: "Test",
+					Name:        "Test",
+					Labels: map[string]string{
+						"traefik.http.routers.Router1.rule":    "Host(`foo.com`)",
+						"traefik.http.routers.Router1.service": "Service1",
+					},
+					NetworkSettings: networkSettings{
+						Ports: nat.PortMap{
+							nat.Port("80/tcp"): []nat.PortBinding{},
+						},
+						Networks: map[string]*networkData{
+							"bridge": {
+								Name: "bridge",
+								Addr: "127.0.0.1",
+							},
+						},
+					},
+				},
+			},
+			expected: &dynamic.Configuration{
+				TCP: &dynamic.TCPConfiguration{
+					Routers:  map[string]*dynamic.TCPRouter{},
+					Services: map[string]*dynamic.TCPService{},
+				},
+				HTTP: &dynamic.HTTPConfiguration{
+					Routers: map[string]*dynamic.Router{
+						"Router1": {
+							Service: "Service1",
+							Rule:    "Host(`foo.com`)",
+						},
+					},
+					Middlewares: map[string]*dynamic.Middleware{},
+					Services: map[string]*dynamic.Service{
+						"Test": {
+							LoadBalancer: &dynamic.ServersLoadBalancer{
+								Servers: []dynamic.Server{
+									{
+										URL: "http://127.0.0.1:80",
+									},
+								},
+								PassHostHeader: true,
+							},
+						},
+					},
+				},
+			},
+		},
 		{
 			desc: "two containers with same service name and different passhostheader",
 			containers: []dockerData{