Update alpine Docker tag to v3.21.0

Dockerfile

@@ -4,7 +4,7 @@ COPY go.mod go.sum ./
 RUN go mod download
 COPY *.go ./
 RUN CGO_ENABLED=0 GOOS=linux go build -o /gotifyWebhookForwarder
-FROM alpine:3.20.3
+FROM alpine:3.21.0
 RUN apk add --no-cache ca-certificates
 COPY --from=build /gotifyWebhookForwarder /gotifyWebhookForwarder
 ENTRYPOINT ["/gotifyWebhookForwarder"]