Update alpine Docker tag to v3.21

| datasource | package | from | to   |
| ---------- | ------- | ---- | ---- |
| docker     | alpine  | 3.20 | 3.21 |


Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Dockerfile

@@ -35,7 +35,7 @@ RUN xx-apt install -y xx-cxx-essentials zlib1g-dev libcurl4-openssl-dev libc-are
 
 
 ### Build base image for alpine
-FROM --platform=$BUILDPLATFORM alpine:3.20 AS build-base-alpine
+FROM --platform=$BUILDPLATFORM alpine:3.21 AS build-base-alpine
 
 RUN apk add --no-cache \
     alpine-sdk \