Do not omit empty json field in /containers/json api response

Signed-off-by: Antonio Murdaca <runcom@linux.com>

api/types/types.go

@@ -101,16 +101,16 @@ type Port struct {
 }
 
 type Container struct {
-	ID         string            `json:"Id"`
-	Names      []string          `json:",omitempty"`
-	Image      string            `json:",omitempty"`
-	Command    string            `json:",omitempty"`
-	Created    int               `json:",omitempty"`
-	Ports      []Port            `json:",omitempty"`
-	SizeRw     int               `json:",omitempty"`
-	SizeRootFs int               `json:",omitempty"`
-	Labels     map[string]string `json:",omitempty"`
-	Status     string            `json:",omitempty"`
+	ID         string `json:"Id"`
+	Names      []string
+	Image      string
+	Command    string
+	Created    int
+	Ports      []Port
+	SizeRw     int
+	SizeRootFs int
+	Labels     map[string]string
+	Status     string
 }
 
 // POST "/containers/"+containerID+"/copy"

integration-cli/docker_api_containers_test.go

@@ -51,6 +51,41 @@ func (s *DockerSuite) TestContainerApiGetAll(c *check.C) {
 	}
 }
 
+// regression test for empty json field being omitted #13691
+func (s *DockerSuite) TestContainerApiGetJSONNoFieldsOmitted(c *check.C) {
+	runCmd := exec.Command(dockerBinary, "run", "busybox", "true")
+	_, err := runCommand(runCmd)
+	c.Assert(err, check.IsNil)
+
+	status, body, err := sockRequest("GET", "/containers/json?all=1", nil)
+	c.Assert(status, check.Equals, http.StatusOK)
+	c.Assert(err, check.IsNil)
+
+	// empty Labels field triggered this bug, make sense to check for everything
+	// cause even Ports for instance can trigger this bug
+	// better safe than sorry..
+	fields := []string{
+		"Id",
+		"Names",
+		"Image",
+		"Command",
+		"Created",
+		"Ports",
+		"SizeRw",
+		"SizeRootFs",
+		"Labels",
+		"Status",
+	}
+
+	// decoding into types.Container do not work since it eventually unmarshal
+	// and empty field to an empty go map, so we just check for a string
+	for _, f := range fields {
+		if !strings.Contains(string(body), f) {
+			c.Fatalf("Field %s is missing and it shouldn't", f)
+		}
+	}
+}
+
 func (s *DockerSuite) TestContainerApiGetExport(c *check.C) {
 	name := "exportcontainer"
 	runCmd := exec.Command(dockerBinary, "run", "--name", name, "busybox", "touch", "/test")