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

Dockerfile

@@ -7,7 +7,7 @@ COPY . /app
 RUN go mod download
 RUN go build ./cmd/asnmap
 
-FROM alpine:3.17.3
+FROM alpine:3.21.0
 RUN apk -U upgrade --no-cache \
     && apk add --no-cache bind-tools ca-certificates
 COPY --from=builder /app/asnmap /usr/local/bin/