fix: add heroku port environment variables binding

Signed-off-by: BoHong Li <raccoon@hackmd.io>

Author: a60814billy

app.json

@@ -15,6 +15,10 @@
             "description": "Let npm also install development build tool",
             "value": "false"
         },
+        "CMD_PORT": {
+            "description": "Listen Port",
+            "value": "$PORT"
+        },
         "CMD_DB_URL": {
             "description": "Database URL",
             "value": "$DATABASE_URL"