Update Dockerfile and build script to support custom installation prefix

rocky9/build.sh

@@ -3,6 +3,7 @@
 set -eu
 
 IMAGE_NAME="${IMAGE_NAME:-rocky9-source-stack}"
+PREFIX="${PREFIX:-/usr/local}"
 SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
 SOURCES_DIR="${SOURCES_DIR:-$SCRIPT_DIR/../sources}"
 FETCH_SCRIPT="$SCRIPT_DIR/../fetch-sources.sh"
@@ -14,5 +15,6 @@ fi
 
 SOURCES_DIR="$SOURCES_DIR" "$FETCH_SCRIPT"
 
-echo "Building image $IMAGE_NAME with podman ..."
-exec podman build --build-context sources="$SOURCES_DIR" --tag "$IMAGE_NAME" "$SCRIPT_DIR"
+echo "Building image $IMAGE_NAME with podman (prefix $PREFIX) ..."
+exec podman build --build-context sources="$SOURCES_DIR" \
+    --build-arg PREFIX="$PREFIX" --tag "$IMAGE_NAME" "$SCRIPT_DIR"