apiv2: handle docker-java clients pulling

When docker-java calls images/create?fromImage=x, it expects two things
for a successful response: that both "error" and "errorDetail" are not
set, and that the "progress" message contains one of five hard-coded
strings ("Download complete" being one of them).

Signed-off-by: Igor Korolev <missterr@gmail.com>

pkg/api/handlers/compat/images.go

@@ -255,12 +255,12 @@ func CreateImageFromImage(w http.ResponseWriter, r *http.Request) {
 	// Success
 	utils.WriteResponse(w, http.StatusOK, struct {
 		Status         string            `json:"status"`
-		Error          string            `json:"error"`
+		Error          string            `json:"error,omitempty"`
 		Progress       string            `json:"progress"`
 		ProgressDetail map[string]string `json:"progressDetail"`
 		Id             string            `json:"id"` // nolint
 	}{
-		Status:         fmt.Sprintf("pulling image (%s) from %s", img.Tag, strings.Join(img.Names(), ", ")),
+		Status:         fmt.Sprintf("pulling image (%s) from %s (Download complete)", img.Tag, strings.Join(img.Names(), ", ")),
 		ProgressDetail: map[string]string{},
 		Id:             img.ID(),
 	})

test/apiv2/10-images.at

@@ -41,7 +41,7 @@ t GET images/$iid/json 200 \
   .Id=sha256:$iid \
   .RepoTags[0]=$IMAGE
 
-t POST "images/create?fromImage=alpine" '' 200
+t POST "images/create?fromImage=alpine" '' 200 .error=null .status~".*Download complete.*"
 
 t POST "images/create?fromImage=alpine&tag=latest" '' 200