Use USTC mirror for Nettle source

debian13/Dockerfile

@@ -460,7 +460,7 @@ RUN curl --fail --location --output gmp.tar.xz \
     && rm -rf src
 
 RUN curl --fail --location --output nettle.tar.gz \
-        https://ftp.gnu.org/gnu/nettle/nettle-3.10.2.tar.gz \
+        https://mirrors.ustc.edu.cn/gnu/nettle/nettle-3.10.2.tar.gz \
     && echo "fe9ff51cb1f2abb5e65a6b8c10a92da0ab5ab6eaf26e7fc2b675c45f1fb519b5  nettle.tar.gz" \
         | sha256sum --check - \
     && mkdir src \