Update alpine Docker tag to v3.21

Author: renovate[bot]

Dockerfile

@@ -7,7 +7,7 @@ ADD hello-world.go /go/src/app
 RUN go build hello-world.go
 
 # We use a Docker multi-stage build here in order that we only take the compiled go executable
-FROM alpine:3.19
+FROM alpine:3.21
 
 LABEL org.opencontainers.image.source="https://github.com/jonashackt/docker-hello-world"