Change default PREFIX to /opt/local

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

debian13/Dockerfile

@@ -23,11 +23,11 @@ RUN sed -i 's|deb.debian.org|mirrors.ustc.edu.cn|g' /etc/apt/sources.list.d/debi
 
 # Install prefix of every dependency, and the Debian multiarch directory name (`gcc -dumpmachine`)
 # its libraries are installed under; build.sh derives the latter from `uname -m`.
-ARG PREFIX=/usr/local
+ARG PREFIX=/opt/local
 ARG MULTIARCH=x86_64-linux-gnu
 
 # Make the compiler, CMake, pkg-config and the shell find earlier dependencies under a prefix other
-# than the default /usr/local. CMake (>= 3.29) takes its default install prefix from the environment.
+# than the default /opt/local. CMake (>= 3.29) takes its default install prefix from the environment.
 ENV PATH=${PREFIX}/bin:${PATH} \
     C_INCLUDE_PATH=${PREFIX}/include \
     CPLUS_INCLUDE_PATH=${PREFIX}/include \