fix for compatibility volume creation

pkg/api/handlers/compat/volumes.go

@@ -93,6 +93,29 @@ func CreateVolume(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// See if the volume exists already
+	existingVolume, err := runtime.GetVolume(input.Name)
+	if err != nil && errors.Cause(err) != define.ErrNoSuchVolume {
+		utils.InternalServerError(w, err)
+		return
+	}
+
+	// if using the compat layer and the volume already exists, we
+	// must return a 201 with the same information as create
+	if existingVolume != nil && !utils.IsLibpodRequest(r) {
+		response := docker_api_types.Volume{
+			CreatedAt:  existingVolume.CreatedTime().Format(time.RFC3339),
+			Driver:     existingVolume.Driver(),
+			Labels:     existingVolume.Labels(),
+			Mountpoint: existingVolume.MountPoint(),
+			Name:       existingVolume.Name(),
+			Options:    existingVolume.Options(),
+			Scope:      existingVolume.Scope(),
+		}
+		utils.WriteResponse(w, http.StatusCreated, response)
+		return
+	}
+
 	if len(input.Name) > 0 {
 		volumeOptions = append(volumeOptions, libpod.WithVolumeName(input.Name))
 	}

pkg/api/server/register_volumes.go

@@ -154,7 +154,9 @@ func (s *APIServer) registerVolumeHandlers(r *mux.Router) error {
 	// parameters:
 	//  - in: body
 	//    name: create
-	//    description: attributes for creating a container
+	//    description: |
+	//      attributes for creating a container.
+	//      Note: If a volume by the same name exists, a 201 response with that volume's information will be generated.
 	//    schema:
 	//      $ref: "#/definitions/DockerVolumeCreate"
 	// produces: