opts: MountOpt: improve error for empty value

Before this patch:

    docker run --rm --mount "" busybox
    invalid argument "" for "--mount" flag: EOF

With this patch:

    docker run --rm --mount "" busybox
    invalid argument "" for "--mount" flag: value is empty

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

opts/mount.go

@@ -22,6 +22,11 @@ type MountOpt struct {
 //
 //nolint:gocyclo
 func (m *MountOpt) Set(value string) error {
+	value = strings.TrimSpace(value)
+	if value == "" {
+		return errors.New("value is empty")
+	}
+
 	csvReader := csv.NewReader(strings.NewReader(value))
 	fields, err := csvReader.Read()
 	if err != nil {

opts/mount_test.go

@@ -109,6 +109,10 @@ func TestMountOptErrors(t *testing.T) {
 	tests := []struct {
 		doc, value, expErr string
 	}{
+		{
+			doc:    "empty value",
+			expErr: "value is empty",
+		},
 		{
 			doc:    "missing tmpfs target",
 			value:  "type=tmpfs",