chore(deps): update alpine docker tag to v3.21.0 (#1438)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Dockerfile.alpine

@@ -13,7 +13,7 @@ COPY . .
 RUN go build -v -o ftpserver
 
 # Preparing the final image
-FROM alpine:3.20.3
+FROM alpine:3.21.0
 WORKDIR /app
 EXPOSE 2121-2130
 COPY --from=builder /workspace/ftpserver /bin/ftpserver