Update alpine Docker tag to v3.21.0 (#398)

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

Dockerfile

@@ -10,6 +10,6 @@ COPY . .
 RUN go install .
 
 # package
-FROM alpine:3.20.3@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a
+FROM alpine:3.21.0@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45
 COPY --from=compile /go/bin/k8s-gitlab-gc /k8s-gitlab-gc
 ENTRYPOINT ["/k8s-gitlab-gc"]