chore(deps): update alpine docker tag to v3.21.0

Author: renovate[bot]

Dockerfile

@@ -14,7 +14,7 @@ RUN go build \
     -o /go/bin/aws-ecr-image-scan-findings-prometheus-exporter \
     -ldflags '-s -w'
 
-FROM alpine:3.18.2 as runner
+FROM alpine:3.21.0 as runner
 
 COPY --from=builder /go/bin/aws-ecr-image-scan-findings-prometheus-exporter /app/aws-ecr-image-scan-findings-prometheus-exporter