Update alpine to 3.21.0 from 3.20.3

Release notes https://alpinelinux.org/posts/Alpine-3.21.0-released.html

Dockerfile

@@ -17,7 +17,7 @@ RUN cmd/bump/main_test.sh /bump
 
 # bump: alpine /FROM alpine:([\d.]+)/ docker:alpine|^3
 # bump: alpine link "Release notes" https://alpinelinux.org/posts/Alpine-$LATEST-released.html
-FROM alpine:3.20.3 AS bump-base
+FROM alpine:3.21.0 AS bump-base
 # git is used by github action code
 # curl for convenience
 RUN apk add --no-cache \