Use PORT instead of UPAAS_PORT for listen port

internal/config/config.go

@@ -89,6 +89,9 @@ func setupViper(name string) {
 	viper.AutomaticEnv()
 
 	// Defaults
+	// PORT is not prefixed with UPAAS_ for compatibility
+	_ = viper.BindEnv("PORT", "PORT")
+
 	viper.SetDefault("PORT", defaultPort)
 	viper.SetDefault("DEBUG", false)
 	viper.SetDefault("DATA_DIR", "./data")