Update alpine Docker tag to v3.21

[skip ci]

Author: renovate[bot]

scripts/docker-compose.bump.yml

@@ -51,7 +51,7 @@ services:
     image: golang:1.23.3-alpine
 
   alpine:
-    image: alpine:3.20
+    image: alpine:3.21
 
   python:
     image: python:3.13.0-alpine