Update alpine Docker tag to v3.21 (#2874)

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

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

Author: renovate[bot]

build/Dockerfile

@@ -11,7 +11,7 @@ RUN make build
 
 FROM golang:1.23 AS ca-certs-provider
 
-FROM alpine:3.20 AS capabilizer
+FROM alpine:3.21 AS capabilizer
 RUN apk add --no-cache libcap
 
 FROM capabilizer AS local-capabilizer

build/Dockerfile.nginxplus

@@ -5,7 +5,7 @@ FROM scratch AS nginx-files
 ADD --link --chown=101:1001 https://cs.nginx.com/static/keys/nginx_signing.rsa.pub nginx_signing.rsa.pub
 
 
-FROM alpine:3.20
+FROM alpine:3.21
 
 ARG NGINX_PLUS_VERSION=R33
 ARG NJS_DIR