build: support selecting environment config file

internal/config/config.go

@@ -93,6 +93,7 @@ type MultiTableTables struct {
 type LoadOptions struct {
 	Environment string
 	ConfigDir   string
+	ConfigFile  string
 }
 
 func Load(options LoadOptions) (Config, error) {
@@ -105,6 +106,9 @@ func Load(options LoadOptions) (Config, error) {
 	if environment == "" {
 		environment = normalizeEnvironment(os.Getenv("APP_ENV"))
 	}
+	if options.ConfigFile != "" && environment == "" {
+		environment = normalizeEnvironment(environmentFromConfigFile(options.ConfigFile))
+	}
 
 	var cfg Config
 	basePath := filepath.Join(configDir, "config.yml")
@@ -112,7 +116,15 @@ func Load(options LoadOptions) (Config, error) {
 		return Config{}, fmt.Errorf("load base config %s: %w", basePath, err)
 	}
 
-	if environment == "test" || environment == "prod" {
+	if options.ConfigFile != "" {
+		profilePath := options.ConfigFile
+		if !filepath.IsAbs(profilePath) {
+			profilePath = filepath.Join(configDir, profilePath)
+		}
+		if err := cleanenv.ReadConfig(profilePath, &cfg); err != nil {
+			return Config{}, fmt.Errorf("load config file %s: %w", profilePath, err)
+		}
+	} else if environment == "test" || environment == "prod" {
 		profilePath := filepath.Join(configDir, "config."+environment+".yml")
 		if err := cleanenv.ReadConfig(profilePath, &cfg); err != nil {
 			return Config{}, fmt.Errorf("load profile config %s: %w", profilePath, err)
@@ -139,6 +151,11 @@ func Load(options LoadOptions) (Config, error) {
 	return cfg, nil
 }
 
+func environmentFromConfigFile(path string) string {
+	name := strings.TrimSuffix(filepath.Base(path), filepath.Ext(path))
+	return strings.TrimPrefix(name, "config.")
+}
+
 func normalizeEnvironment(value string) string {
 	switch strings.ToLower(strings.TrimSpace(value)) {
 	case "production", "prod":

internal/config/config_test.go

@@ -51,6 +51,24 @@ auth:
 	}
 }
 
+func TestLoadExplicitConfigFile(t *testing.T) {
+	dir := writeConfigs(t, `
+app:
+  env: test
+server:
+  port: 8081
+database:
+  name: explicit_db
+`)
+	cfg, err := Load(LoadOptions{ConfigDir: dir, ConfigFile: "config.test.yml"})
+	if err != nil {
+		t.Fatalf("Load() error = %v", err)
+	}
+	if cfg.App.Env != "test" || cfg.Server.Port != 8081 || cfg.Database.Name != "explicit_db" || cfg.Database.User != "base_user" {
+		t.Fatalf("unexpected explicit config: %+v", cfg)
+	}
+}
+
 func TestLoadProductionRequiresEnvironmentSecrets(t *testing.T) {
 	dir := writeConfigs(t, "app:\n  env: production\n")
 	t.Setenv("APP_DATABASE_PASSWORD", "")