Use USTC EPEL mirror and upgrade packages in rocky9 Dockerfile

Port the EPEL mirror rewrite from develop's test/Dockerfile, and run
dnf upgrade before cleaning the cache.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012LvSM3NjBSCNsRwDpWeGfu

rocky9/Dockerfile

@@ -23,8 +23,14 @@ RUN dnf install --assumeyes --enablerepo=crb \
         xz \
         which \
         epel-release \
+    && sed \
+        -e 's!^metalink=!#metalink=!g' \
+        -e 's!^#baseurl=!baseurl=!g' \
+        -e 's!https\?://download\.example/pub/epel!https://mirrors.ustc.edu.cn/epel!g' \
+        -i /etc/yum.repos.d/epel.repo /etc/yum.repos.d/epel-testing.repo \
     && dnf install --assumeyes \
         makeself \
+    && dnf upgrade --assumeyes \
     && dnf clean all \
     && rm -rf /var/cache/dnf