MM-47238 Add MM_BOARDS_DEV_SERVER_URL environment variable for MPA (#21435)

* MM-47238 Add MM_BOARDS_DEV_SERVER_URL environment variable for MPA

* Switch from os.LookupEnv to os.Getenv

web/handlers.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"os"
 	"reflect"
 	"runtime"
 	"strconv"
@@ -133,8 +134,13 @@ func generateDevCSP(c Context) string {
 
 	// Add flags for Webpack dev servers used by other products during development
 	if model.BuildNumber == "dev" {
-		// Focalboard runs on http://localhost:9006
-		devCSP = append(devCSP, "http://localhost:9006")
+		boardsURL := os.Getenv("MM_BOARDS_DEV_SERVER_URL")
+		if boardsURL == "" {
+			// Focalboard runs on http://localhost:9006 by default
+			boardsURL = "http://localhost:9006"
+		}
+
+		devCSP = append(devCSP, boardsURL)
 	}
 
 	if len(devCSP) == 0 {