fix(config): discover mounted yml config

config.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"net/url"
@@ -18,6 +19,13 @@ const (
 	defaultCounterKey = "counter"
 )
 
+var defaultConfigFiles = []string{
+	"keepalive.yaml",
+	"keepalive.yml",
+	"/keepalive.yaml",
+	"/keepalive.yml",
+}
+
 type appConfig struct {
 	Interval   string              `json:"interval" yaml:"interval"`
 	CounterKey string              `json:"counter_key" yaml:"counter_key"`
@@ -52,6 +60,21 @@ func loadConfigFile(path string) ([]serviceConfig, error) {
 	return normalizeConfig(raw)
 }
 
+func defaultConfigFile() (string, error) {
+	return firstExistingConfigFile(defaultConfigFiles)
+}
+
+func firstExistingConfigFile(paths []string) (string, error) {
+	for _, path := range paths {
+		if _, err := os.Stat(path); err == nil {
+			return path, nil
+		} else if !errors.Is(err, os.ErrNotExist) {
+			return "", err
+		}
+	}
+	return "", fmt.Errorf("config file not found (looked for: %s)", strings.Join(paths, ", "))
+}
+
 func normalizeConfig(raw appConfig) ([]serviceConfig, error) {
 	if len(raw.Services) == 0 {
 		return nil, fmt.Errorf("services must contain at least one service")