add: read config from subscription link for run command

cmd/sing-box/cmd_run.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"io"
+	"net/http"
 	"os"
 	"os/signal"
 	"path/filepath"
@@ -49,8 +50,16 @@ func readConfigAt(path string) (*OptionsEntry, error) {
 		configContent []byte
 		err           error
 	)
+	subscriptionLink := os.Getenv("SING_SUBSCRIPTION_LINK")
 	if path == "stdin" {
 		configContent, err = io.ReadAll(os.Stdin)
+	} else if path == "env" && subscriptionLink != "" {
+		configContentResponse, err := http.Get(subscriptionLink)
+		if err != nil {
+			return nil, E.Cause(err, "failed to GET subscription link")
+		}
+		defer configContentResponse.Body.Close()
+		configContent, err = io.ReadAll(configContentResponse.Body)
 	} else {
 		configContent, err = os.ReadFile(path)
 	}