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

Author: renovate[bot]

alpine.dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.20@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a as builder
+FROM alpine:3.21@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45 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@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a
+FROM alpine:3.21@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45
 
 RUN apk --no-cache add \
     libcurl \