Update alpine Docker tag to v3.21 (#64)

Co-authored-by: elastic-renovate-prod[bot] <174716857+elastic-renovate-prod[bot]@users.noreply.github.com>

Author: elastic-renovate-prod[bot]

Dockerfile

@@ -5,7 +5,7 @@ RUN apk --no-cache add git
 RUN CGO_ENABLED=0 go install go.k6.io/xk6/cmd/xk6@latest
 RUN CGO_ENABLED=0 xk6 build --with github.com/elastic/xk6-output-elasticsearch=. --output /tmp/k6
 
-FROM alpine:3.20@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a
+FROM alpine:3.21@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45
 RUN apk add --no-cache ca-certificates && \
     adduser -D -u 12345 -g 12345 k6
 COPY --from=builder /tmp/k6 /usr/bin/k6