Build Protobuf from source in rocky9-system

Every other library still comes from the distribution packages.

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

rocky9-system/build.sh

@@ -4,16 +4,16 @@ set -eu
 
 IMAGE_NAME="${IMAGE_NAME:-rocky9-system-stack}"
 SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+SOURCES_DIR="${SOURCES_DIR:-$SCRIPT_DIR/../sources}"
+FETCH_SCRIPT="$SCRIPT_DIR/../fetch-sources.sh"
 
-if command -v podman >/dev/null 2>&1; then
-    ENGINE=podman
-elif command -v docker >/dev/null 2>&1; then
-    ENGINE=docker
-else
-    echo "Error: Podman or Docker is required." >&2
+if ! command -v podman >/dev/null 2>&1; then
+    echo "Error: Podman is required." >&2
     exit 1
 fi
-ENGINE="${CONTAINER_ENGINE:-$ENGINE}"
 
-echo "Building image $IMAGE_NAME with $ENGINE ..."
-exec "$ENGINE" build --tag "$IMAGE_NAME" "$SCRIPT_DIR"
+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"