chore(deps): update alpine docker tag to v3.21

Author: renovate[bot]

alpine.dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.20@sha256:31687a2fdd021f85955bf2d0c2682e9c0949827560e1db546358ea094f740f12 AS builder
+FROM alpine:3.21@sha256:a8560b36e8b8210634f77d9f7f9efd7ffa463e380b75e2e74aff4511df3ef88c AS builder
 ARG VERSION
 
 RUN apk --no-cache add \
@@ -26,7 +26,7 @@ RUN ./configure \
 RUN make
 RUN make install DESTDIR=/build/output
 
-FROM alpine:3.20@sha256:31687a2fdd021f85955bf2d0c2682e9c0949827560e1db546358ea094f740f12
+FROM alpine:3.21@sha256:a8560b36e8b8210634f77d9f7f9efd7ffa463e380b75e2e74aff4511df3ef88c
 
 RUN apk --no-cache add \
     libcurl \