build: support selecting environment config file

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", "")