envmap := envVarsToMap(envs)
	missingKeys := sets.New[string]()
	expanded := expansion.Expand(mount.SubPathExpr, func(key string) string {
		value, ok := envmap[key]
		if !ok || len(value) == 0 {
			missingKeys.Insert(key)
		}
		return value
	})

	if len(missingKeys) > 0 {
		return "", fmt.Errorf("missing value for %s", strings.Join(sets.List(missingKeys), ", "))
	}
	return expanded, nil
}

// value of the pipeline, if any.
func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node, args []parse.Node, final, receiver reflect.Value) reflect.Value {
	if !receiver.IsValid() {
		if s.tmpl.option.missingKey == mapError { // Treat invalid value as missing map key.
			s.errorf("nil data; no entry for key %q", fieldName)
		}
		return zero
	}
	typ := receiver.Type()
	receiver, isNil := indirect(receiver)