build: add migration image bundling to build pipeline

Bundle start9/compat, start9/utils, and tonistiigi/binfmt container
images into the OS image so the v1→v2 s9pk migration can run without
internet access.

build/save-migration-images.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+# Save Docker images needed by the 0.3.6-alpha.0 migration as tarballs
+# so they can be bundled into the OS and loaded without internet access.
+set -e
+
+ARCH="${ARCH:-x86_64}"
+DESTDIR="${1:-build/lib/migration-images}"
+
+if [ "$ARCH" = "x86_64" ]; then
+    DOCKER_PLATFORM="linux/amd64"
+elif [ "$ARCH" = "aarch64" ]; then
+    DOCKER_PLATFORM="linux/arm64"
+else
+    DOCKER_PLATFORM="linux/$ARCH"
+fi
+
+IMAGES=("start9/compat:latest" "start9/utils:latest" "tonistiigi/binfmt:latest")
+
+mkdir -p "$DESTDIR"
+
+for IMAGE in "${IMAGES[@]}"; do
+    FILENAME=$(echo "$IMAGE" | sed 's|/|_|g; s/:/_/g').tar
+    if [ -f "$DESTDIR/$FILENAME" ]; then
+        echo "Skipping $IMAGE (already saved)"
+        continue
+    fi
+    echo "Pulling $IMAGE for $DOCKER_PLATFORM..."
+    docker pull --platform "$DOCKER_PLATFORM" "$IMAGE"
+    echo "Saving $IMAGE to $DESTDIR/$FILENAME..."
+    docker save "$IMAGE" -o "$DESTDIR/$FILENAME"
+done
+
+echo "Migration images saved to $DESTDIR"