Update Dockerfile and build script to support custom installation prefix

README.md

@@ -19,6 +19,13 @@ Use environment variables to select the container engine or override image names
 CONTAINER_ENGINE=podman IMAGE_NAME=my-build-env ./rocky9/build.sh
 ```
 
+The Debian 13 and Rocky Linux 9 images install every library under `/usr/local` by default. Set `PREFIX` to install them elsewhere; the build environment variables (`PATH`, `PKG_CONFIG_PATH`, `CMAKE_PREFIX_PATH`, `CMAKE_INSTALL_PREFIX`, and so on) follow it. The Alpine image does not support `PREFIX`.
+
+```sh
+PREFIX=/opt/stack ./debian13/build.sh
+PREFIX=/opt/stack ./rocky9/build.sh
+```
+
 ## Run
 
 ```sh