Support run mode set by env var BEEGO_RUNMODE

@@ -26,6 +26,7 @@ import (
 	"github.com/astaxie/beego/logs"
 	"github.com/astaxie/beego/session"
 	"github.com/astaxie/beego/utils"
+	"github.com/kelseyhightower/envconfig"
 )
 
 var (
@@ -83,9 +84,14 @@ var (
 	RouterCaseSensitive    bool   // router case sensitive default is true
 )
 
-type beegoAppConfig struct {
-	innerConfig config.ConfigContainer
-}
+type (
+	beegoAppConfig struct {
+		innerConfig config.ConfigContainer
+	}
+	beegoEnvConfig struct {
+		RunMode string
+	}
+)
 
 func newAppConfig(AppConfigProvider, AppConfigPath string) *beegoAppConfig {
 	ac, err := config.NewConfig(AppConfigProvider, AppConfigPath)
@@ -288,8 +294,16 @@ func init() {
 func ParseConfig() (err error) {
 	AppConfig = newAppConfig(AppConfigProvider, AppConfigPath)
 
+	var ec beegoEnvConfig
+	err = envconfig.Process("beego", &ec)
+	if err != nil {
+		return err
+	}
+
 	// set the runmode first
-	if runmode := AppConfig.String("RunMode"); runmode != "" {
+	if ec.RunMode != "" {
+		RunMode = ec.RunMode
+	} else if runmode := AppConfig.String("RunMode"); runmode != "" {
 		RunMode = runmode
 	}