Update alpine Docker tag to v3.21 (#206)

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

Author: renovate[bot]

Dockerfile

@@ -3,7 +3,7 @@ WORKDIR /go/src/app
 COPY . /go/src/app
 RUN go build -o ./kube-job-notifier *.go
 
-FROM alpine:3.19
+FROM alpine:3.21
 LABEL maintainer="yutachaos <bumplive@gmail.com>"
 
 COPY --from=build-env /go/src/app/kube-job-notifier .